Added contribution guidelines

CONTRIBUTING.md

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+# Contributing
+
+## Important notes
+Please don't edit files in the `dist` subdirectory as they are generated via Grunt. You'll find source code in the `src` subdirectory!
+
+### Code style
+Regarding code style like indentation and whitespace, **follow the conventions you see used in the source already.**
+
+### PhantomJS
+While Grunt can run the included unit tests via [PhantomJS](http://phantomjs.org/), this shouldn't be considered a substitute for the real thing. Please be sure to test the `test/*.html` unit test file(s) in _actual_ browsers.
+
+## Modifying the code
+First, ensure that you have the latest [Node.js](http://nodejs.org/) and [npm](http://npmjs.org/) installed.
+
+Test that Grunt's CLI is installed by running `grunt --version`.  If the command isn't found, run `npm install -g grunt-cli`.  For more information about installing Grunt, see the [getting started guide](http://gruntjs.com/getting-started).
+
+1. Fork and clone the repo.
+1. Run `npm install` to install all dependencies (including Grunt).
+1. Run `grunt` to grunt this project.
+
+Assuming that you don't see any red, you're ready to go. Just be sure to run `grunt` after making any changes, to ensure that nothing is broken.
+
+## Submitting pull requests
+
+1. Create a new branch, please don't work in your `master` branch directly.
+1. Add failing tests for the change you want to make. Run `grunt` to see the tests fail.
+1. Fix stuff.
+1. Run `grunt` to see if the tests pass. Repeat steps 2-4 until done.
+1. Open `test/*.html` unit test file(s) in actual browser to ensure tests pass everywhere.
+1. Update the documentation to reflect any changes.
+1. Push to your fork and submit a pull request.