Create a contributing guide for the repo.

@LiamCottrell Love the idea, before we merge can you create a contributing guide for the repo? Avoid having to look up what to do with all of this post-merge?

_Originally posted by @TerenceJefferies in https://github.com/TerenceJefferies/STime/pull/1#issuecomment-650761749_

The contribution file will also detail the PR process for the repository and responsibilities of contributors. 