Docs: Add CONTRIBUTING.md (fixes #9)

CONTRIBUTING.md

@@ -0,0 +1,21 @@
+# Contributing
+
+Thanks for contributing!
+
+## Installation
+
+```bash
+git clone https://github.com/not-an-aardvark/eslint-plugin-prettier.git
+cd eslint-plugin-prettier
+npm install
+```
+
+## Running the tests
+
+```bash
+npm test
+```
+
+This is an [ESLint](http://eslint.org) plugin. Documentation for the APIs that it uses can be found on ESLint's [Working with Plugins](http://eslint.org/docs/developer-guide/working-with-plugins) page.
+
+This plugin is used to lint itself. The style is checked when `npm test` is run, and the build will fail if there are any linting errors. You can use `npm run lint -- --fix` to fix some linting errors. To run the tests without running the linter, you can use `node_modules/.bin/mocha tests --recursive`.

README.md

@@ -72,3 +72,7 @@ To install `prettier`, use:
 ```bash
 npm install prettier --save-dev
 ```
+
+## Contributing
+
+See [CONTRIBUTING.md](https://github.com/not-an-aardvark/eslint-plugin-prettier/blob/master/CONTRIBUTING.md)