CONTRIBUTING.md Documentation

Created by: Nolski

As noted by @kfogel in a recent PR it seems that there are certain style requirements required by maintainers (such as linking to resolved issues in commit messages instead of PR text). I think it would be useful to lower the barrier of entry for new contributors looking to submit a patch by having this documented somewhere in the root of the repository or README.

I propose we use this issue as a place to discuss what requirements we might have for new contributors to this project & where they should live.