Add CONTRIBUTING.md

[adamghill]

Looking at https://gist.github.com/PurpleBooth/b24679402957c63ec426 (although most of that is a CoC and I already have a separate CoC already) and https://github.com/django/django/blob/main/CONTRIBUTING.rst (although that links to a lot of other docs about how to contribute to Django).

@mihrab34 Do you have an example CONTRIBUTING file that you seen in the past and liked?

[mihrab34]

@adamghill I liked this one Async API uses for their projects https://github.com/asyncapi/.github/blob/master/CONTRIBUTING.md

[adamghill]

THis got merged in, so closing this.