Add a little paragraph on “why”

Readme.md

@@ -5,7 +5,7 @@
 [![Build status][travis-image]][travis-url]
 [![Test coverage][coveralls-image]][coveralls-url]
 
-Return a pluralized or singularized word based on the input string.
+> Pluralize and singularize any word.
 
 ## Installation
 
@@ -34,6 +34,10 @@ define(function (require, exports, module) {
 <script src="pluralize.js"></script>
 ```
 
+## Why?
+
+This module uses a pre-defined list of rules, applied in order, to singularize or pluralize a given word. There are many cases where this is useful, such as any automation based on user input. For applications where the word(s) are known ahead of time, you can use a simple ternary (or function) which would be a much lighter alternative.
+
 ## Usage
 
 ```javascript