Moving contrib guidelines into CONTRIBUTING.md

Moving these guidelines into a dedicated file lets GitHub [provide some UI sugar](https://github.com/blog/1184-contributing-guidelines) during PRs. Screenshot of this very PR before I submitted is included.

![github-contributing-pr](https://f.cloud.github.com/assets/254753/433793/559a6746-aef2-11e2-9126-5420b82ac186.png)