Add first draft of a rule description style guide

.github/contributing/rule-description-style.md

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+# Style guide for authoring rule descriptions
+
+A rule incorporated into the Detekt framework is generally accompanied by three
+types of descriptions:
+1. **Documentation string**: Using conventional
+   Javadoc/KDoc syntax, the documentation string is associated with the `Rule`
+   subclass implementing the considered rule. Documentation strings associated
+   with built-in rules are automatically pulled from the Detekt codebase and
+   used to generate the rule set overview available on the
+   [Detekt website](https://detekt.github.io/detekt).
+2. **Issue description**: The issue description gives a summary of the code
+   smells that the respective rule is supposed to detect. From an implementation
+   point of view, it is the string that `Rule` subclasses pass to the
+   `description` parameter of the [`Issue` class][1]. In generated reports, the
+   issue description is often used to introduce a list of code smells that the
+   respective rule has identified.
+3. **Code smell messages**: A code smell message is issued for every code
+   smell (or *violation*) identified within the codebase. Implementation-wise,
+   such a message is dynamically created during the construction of a
+   [`CodeSmell` instance][2]. In generated reports, these messages are listed
+   underneath the issue description. Built-in console reports such as
+   `LiteFindingsReport` use these messages to display identified code smells to
+   the user.
+
+When authoring these descriptions, rule developers should keep two different
+target audiences in mind:
+1. The **documentation string** is generally read by individuals who want to
+   learn about the *rule itself*. They might need to compose a Detekt
+   configuration for their codebase, need to understand what the rule is
+   currently checking for in order to extend it, or are just interested in the
+   available Detekt rule sets.
+2. The **issue description** as well as **code smell messages** are presented to
+   developers whose codebase violates one or more rules that Detekt checked for.
+   This audience is generally less interested in the rule itself. Instead,
+   individuals reading these texts will usually expect specific references to
+   *their codebase* and what can be done in order to improve it.
+
+## Guidelines for documentation strings
+
+When authoring the documentation string for a rule, adhere to the following
+guidelines:
+1. The documentation string shall be formulated in such a manner that it refers
+   to the rule itself:
+   - :heavy_check_mark:: `Detects trailing spaces and recommends to remove them.`
+   - :x:: ``Trailing spaces detected. Consider removing them.``
+2. Stick to the [KDoc convention][3] that the first paragraph of documentation
+   text is the summary of the documented element (the rule), while the following
+   text is a detailed description. More specifically, populate these two regions
+   as follows:
+   - In the **summary**, give a concise description of the violation(s) that the
+   rule identifies. If applicable, a very brief summary of why the violation is
+   considered bad practice or what is usually done to eliminate it can be
+   given (but does not have to be given).
+   - In the **detailed description**, which does not have to be added for
+   simple/obvious rules, explain the violation(s) that the rule identifies in
+   greater detail. Add a rationale (ideally with a reference) outlining why such
+   a violation should be avoided. If applicable, add further remarks that make
+   it easier to understand or configure the rule.
+3. Formulate all descriptions using (potentially incomplete) sentences with
+   proper capitalization and punctuation:
+   - :heavy_check_mark:: `Ensures that files with a single non-private class are named accordingly.`
+   - :x:: `ensures that files with a single non-private class are named accordingly`
+4. Surround inline code (or code symbols) with `` ` `` characters (as in
+   [Markdown][4]):
+   - :heavy_check_mark:: ``Reports referential equality checks for types such as `String` and `List`.``
+   - :x:: `Reports referential equality checks for types such as String and List.`
+
+## Guidelines for issue descriptions and code smell messages
+
+Adhere to the following guidelines when authoring an issue description or code
+smell messages:
+1. Use a wording that focuses on the violation rather than on the rule itself.
+   Therefore, assume that the analyzed codebase contains at least one violation
+   of the respective rule.
+   - :heavy_check_mark:: ``Duplicated case statements in `when` expression.``
+   - :x:: `Detects trailing spaces.`
+2. Formulate the text as a sequence of (potentially incomplete) sentences with
+   proper capitalization and punctuation:
+   - :heavy_check_mark:: ``Duplicated case statements in `when` expression.``
+   - :x:: ``duplicated case statements in `when` expression``
+3. Separate sentences from the sequence with one space. Most importantly, do not
+   use line breaks:
+   - :heavy_check_mark:: `Non-boolean property with prefix suggesting boolean type. This might mislead clients of the API.`
+   - :x:: `Non-boolean property with prefix suggesting boolean type.\nThis might mislead clients of the API.`
+4. Surround inline code (or code symbols) with `` ` `` characters (as in
+   [Markdown][4]):
+   - :x:: `map.get() used with non-null assertion operator (!!).`
+   - :heavy_check_mark:: `` `map.get()` used with non-null assertion operator `!!`.``
+
+*Note*: `non-null` is a borderline case. While `null` itself can be seen as
+code, the prefix turns it into a conventional word.
+
+The above-mentioned guidelines tell you *how* to formulate issue descriptions
+and and code smell messages. To learn *what to include* in each of the texts,
+refer to the following subsections.
+
+### Components of issue descriptions
+An issue description should consist of the following components (in this order):
+1. **Violation** (*required*): What type of violation is it that Detekt
+   identified in the codebase and, under consideration of the given Detekt
+   configuration, recognizes as a violation of the considered rule?
+2. **Rationale** (*optional*): Why is the identified violation considered an
+   issue? Especially for less experienced Kotlin developers, this might not be
+   clear from just the violation itself.
+3. **Recommendation** (*optional*): What are developers usually expected to do
+   in order to eliminate the identified violation? In some cases, this might be
+   entirely obvious. If it is not, however, it makes sense to add a suitable
+   recommendation (or multiple alternative recommendations).
+
+:warning: In any case, try to keep the description as brief and concise as
+possible!
+
+The following list contains some positive and negative examples:
+- :heavy_check_mark: (*just a violation*): `Public library class.`
+- :x: (*just a recommendation*): `Library class should not be public.`
+- :heavy_check_mark: (*violation and recommendation*): ``Public library class. Reduce its visibility to the module or the file.``
+- :x: (*just a rationale*): `A function that only returns a constant is misleading.`
+- :heavy_check_mark: (*violation and rationale*): `Function returns a constant. This is misleading.`
+- :heavy_check_mark: (*all categories*): `Function returns a constant, which is misleading. Use a constant property instead.`
+
+### Components of code smell messages
+A code smell message should be dynamically created to describe a
+*specific violation*. It should be regarded as an extension of the more generic
+violation part of the issue description. More specifically, it should explicitly
+reference identifiers, file names, or similar from the codebase. If it makes
+sense in the specific context, it might be worthwhile to add a more detailed
+version of the recommendation as well. If this is the case, add this extension
+after the more specific description of the violation.
+
+:warning: Try to keep code smell messages even shorter than issue descriptions!
+
+The following list contains compliant and non-compliant examples of code smell
+messages:
+- :x: (*probably not more specific than the issue message*): ``Non-boolean property suggests a boolean type.``
+- :heavy_check_mark: (*specific violation*): ``Non-boolean property `hasXyz` suggests a boolean type.``
+- :x: (*pointless recommendation*): ``Non-boolean property `hasXyz` suggests a boolean type. Remove the prefix.``
+- :heavy_check_mark: (*with specific recommendation*): ``Non-boolean property `hasXyz` suggests a boolean type. Remove the `has` prefix.``
+- :heavy_check_mark: (*with usage-aware recommendation*): ``Magic number `4` passed to `abc`. Pass it as a named argument.``
+
+[1]: https://github.com/detekt/detekt/blob/v1.19.0/detekt-api/src/main/kotlin/io/gitlab/arturbosch/detekt/api/Issue.kt
+[2]: https://github.com/detekt/detekt/blob/v1.19.0/detekt-api/src/main/kotlin/io/gitlab/arturbosch/detekt/api/CodeSmell.kt
+[3]: https://kotlinlang.org/docs/kotlin-doc.html
+[4]: https://daringfireball.net/projects/markdown/syntax