Added contribution guidelines

Author: prsimp

CONTRIBUTING.md

@@ -0,0 +1,39 @@
+# Contributing to csscss
+
+First of all: thanks!
+
+The main way to contribute to csscss is to write some code! Here's how:
+
+1. Fork csscss
+2. Create a topic branch - `git checkout -b my_branch`
+3. Push to your branch - `git push origin my_branch`
+4. Create a [Pull Request](http://help.github.com/pull-requests/) from your
+   branch
+5. That's it!
+
+If you're not doing some sort of refactoring, a CHANGELOG entry is appropriate.
+Please include them in pull requests adding features or fixing bugs.
+
+## Tests
+
+csscss uses minitest for testing.
+
+A simple `bundle exec rake` will run all the tests. Make sure they pass when
+you submit a pull request.
+
+Please include tests with your pull request.
+
+## Bugs & Feature Requests
+
+You can file bugs on the [issues
+tracker](https://github.com/zmoazeni/csscss/issues), and tag them with 'bug'. Feel free to discuss features there, too.
+
+### Good report structure
+
+Please include the following four things in your report:
+
+1. What you did.
+2. What you expected to happen.
+3. What happened instead.
+
+The more information the better.