Create process.rst

[orangejenny]

@ctsims Does this seem useful? It's a bit vague. In practice people submitting non-trivial changes are usually working with an internal contact, so my intent is just for this to help orient people.

[ctsims]

Made some suggestions on wording, mostly since I think it's much better when possible to avoid directly referencing individual codebases for reference (and to push docs into that codebase if unavoidable).

One thought: It might be the case that this doc primarily wants to communicate about QA, and should be focused there instead of a broader contribution doc. IE: Adding a QA doc to docs/ and updating the template at https://github.com/dimagi/open-source/blob/master/templates/CONTRIBUTING.rst to reference it.

[orangejenny]

@ctsims Cool, thanks for feedback. I'm going to leave it framed as a general process doc, because I do want it to be broader than QA. The label process in particular has been a point of confusion, since if you don't have write access your PR immediately fails that check.