Create CONTRIBUTING.md

CONTRIBUTING.md

+Contributing
+============
+
+Want to contribute? Great! First, read this page.
+
+# Code reviews
+All submissions, including submissions by project members, require review. We
+use Github pull requests for this purpose.
+
+# Some tips for good pull requests:
+* Use our code
+  When in doubt, try to stay true to the existing code of the project.
+* Write a descriptive commit message. What problem are you solving and what
+  are the consequences? Where and what did you test? Some good tips:
+  [here](http://robots.thoughtbot.com/5-useful-tips-for-a-better-commit-message)
+  and [here](https://www.kernel.org/doc/Documentation/SubmittingPatches).
+* If your PR consists of multiple commits which are successive improvements /
+  fixes to your first commit, consider squashing them into a single commit
+  (`git rebase -i`) such that your PR is a single commit on top of the current
+  HEAD. This make reviewing the code so much easier, and our history more
+  readable.
+
+# Formatting
+
+This documentation is written using standard [markdown syntax](https://help.github.com/articles/markdown-basics/). Please submit your changes using the same syntax.