document "how to become a maintainer" and "how to be a good maintainer"

[boneskull]

This probably exists in a "project" somewhere, but we need to formalize contribution guidelines, decision-making process, etc. I like what ESLint has done.

UPDATE: I forgot that I already wrote up CONTRIBUTING.md, and I can live with that for now. What we're missing is "how to become a maintainer" and "how to be a good maintainer once you are one", etc.

I'll tackle "governance" in another issue.

[boneskull]

there is a bit of "governance" in here, but I don't think we have enough "governance" to warrant a separate doc for it (yet)