Add contribution guidelines

Author: calebhearth

CONTRIBUTING.md

@@ -0,0 +1,12 @@
+# Contributing
+
+1. Fork it
+2. Create your feature branch (`git checkout -b my-new-feature`)
+3. Add your tests
+4. Add your feature or bugfix
+5. Run the tests. We only accept pull requests with passing tests.
+6. Commit your changes (`git commit -am 'Add some feature'`)
+7. Make sure your changes adhere to the
+   [thoughtbot Style Guide](https://github.com/thoughtbot/guides/tree/master/style)
+8. Push to the branch (`git push origin my-new-feature`)
+9. Create new Pull Request