Org-level CONTRIBUTING.md and MAINTAINERS.md

[lbordowitz]

Every supervised repository (see #23) should have these two guides describing how to contribute to it, other projects that might be contributed to, and who maintains these repositories. While these files don't need to be the same in every single repo, any fixes done to some central CONTRIBUTING.md and MAINTAINERS.md should be reflected everywhere.

We may also want to list different ways to contribute that may not include code, or auxiliary contributions to the main repo (like CI/CD).