Add Contributing guide

.github/CONTRIBUTING.md

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+# Contributing to http-server
+
+> Please read these guidelines before submitting an issue, filing a feature request, or contributing code.
+
+## :bug: I Found a Bug
+
+Sorry! It happens to the best of us. If you've found a bug in http-server, **please [search](https://github.com/http-party/http-server/issues/) to see if it's already been reported**. Otherwise, create a [new issue](https://github.com/http-party/http-server/issues/new). If you can fix the bug yourself, feel free to create a [pull request](#propose-a-change) thereafter.
+
+Please include _as much detail as possible_ to help us reproduce and diagnose the bug. Most importantly:
+
+- Make use of the issue template!
+- Let us know _how_ you're running http-server (options, flags, environment, etc.)
+- Include your test code or file(s). If large, please provide a link to a repository or [gist](https://gist.github.com).
+- Please show code in JavaScript only (any version)
+
+If we need more information from you, we'll let you know. If you don't within a reasonable time frame (TBD), your issue will be automatically tagged as stale and eventually closed for inactivity.
+
+## :exclamation: Propose a Change
+
+Before you get your hands dirty, please [search](https://github.com/http-party/http-server/issues/) for a related issue, or [create a new one](https://github.com/http-party/http-server/issues/new). If you wish to contribute a new feature, this is doubly important! Let's discuss your proposed changes first; we don't want you to waste time implementing a change that is at odds with the project's direction. That said, we'll happily consider any contribution, no matter how great or small.
+
+### :shoe: Contributing Code: Step-by-Step
+
+Follow these steps to get going.
+
+1. [Install the latest version of Node.js](https://nodejs.org/en/download).
+    - If you're new to installing Node, a tool like [nvm](https://github.com/creationix/nvm#install-script) can help you manage multiple version installations.
+1. Follow [Github's documentation](https://help.github.com/articles/fork-a-repo/) on setting up Git, forking and cloning.
+1. Create a new branch in your fork, giving it a descriptive name
+1. Execute `npm install` to install the prod and dev dependencies
+   - Do not use `yarn install` for development, as it may not get the same package versions as other developers.
+1. Make your changes and add them via `git add`.
+   - **Tests are required** for any non-trivial code change. If you're having trouble making tests, go ahead and open the pull request and we can help
+   - Keep your PR focused. Don't fix multiple things at once, and don't upgrade dependencies unless necessary.
+1. Before committing, run `npm test`
+   - Tests will also run on your PR, but running them locally will let you catch problems ahead-of-time.
+1. Commit your changes.
+   - See [How to Write a Git Commit Message](https://chris.beams.io/posts/git-commit/).
+1. Push your changes to your fork.
+1. Now on [http-party/http-server](https://github.com/http-party/http-server), you should see a notification about your recent changes in your fork's branch, with a green button to create a pull request. Click the button.
+1. Describe your changes in detail here, following the template. Once you're satisfied, submit the form.
+1. Be patient while your PR is reviewed. This can take a while. We may request changes, but don't be afraid to question them.
+1. Your PR might become conflicted with the code in `master`. If this is the case, you will need to [update your PR](#up-to-date) and resolve your conflicts.
+1. You don't need to make a new PR to any needed changes. Instead, commit on top of your changes, and push these to your fork's branch. The PR will be updated, and CI will re-run.
+   - **Please do not rebase and force-push**, it ruins the git history
+
+## :angel: I Just Want To Help
+
+_Excellent._ Here's how:
+
+- **Handy with JavaScript?** Please check out the issues labeled [`help-wanted`](https://github.com/http-party/http-server/issues?q=is%3Aopen+is%3Aissue+label%3A%22help-wanted%22) or [`good first issue`](https://github.com/http-party/http-server/issues?q=is%3Aissue+is%3Aopen+sort%3Aupdated-desc+label%3Agood+first+issue). 
+- **Wait--you write unit tests for _fun_?** A PR which increases coverage is unlikely to ever be turned down.