Add contributing guidelines

Author: amacneil

CONTRIBUTING.md

@@ -0,0 +1,10 @@
+# Contributing Guidelines
+
+* Fork the project.
+* Make your feature addition or bug fix.
+* Add tests for it. This is important so I don't break it in a future version unintentionally.
+* Commit just the modifications, do not mess with the composer.json or CHANGELOG.md files.
+* Ensure your code is nicely formatted in the [PSR-2](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md)
+  style and that all tests pass.
+* Send the pull request.
+* Check that the Travis CI build passed. If not, rinse and repeat.