Document how to contribute more?

We have quite nice documentation for first-time contributors, but (seem to have) nothing on the process by which keen ones could become more involved. Right now, the procedure seems to be that someone gets recognized by one of the maintainers. Would it make sense to spell it out a bit? E.g., some sense of the number of substantial PRs; the ability to self-nominate?

Somewhat related: would it make sense to have on our team page a "help wanted" category? (e.g., I'd happily somewhat reduce my role in `time` and `table`)

Review the existing documentation for first-time contributors and the project team page first. Define whether the project wants a documented path for becoming more involved, including substantial PRs and self-nomination, and whether a help-wanted category belongs on the team page; done means the maintainers agree on and publish a clear process.

Written by the indexing model from the issue text.