Define `github` practices

This may include:
* PR workflow used (e.g. git-flow, github flow, OR just allowing it to be at the maintainer's discretion)
* Expectations on the size and focus of a PR
* Expectations on being prudent with our GH minutes (maybe not important with all public repositories, but I'm thinking something like "test before commit" and "check before push" on our R packages)
* Merge strategies see #17 