Decision: Mechanisms for adding and removing maintainers

Once we've resolved #143 and have agreed on a voting mechanism, we need to write down the process for adding (and removing, but adding is more relevant) maintainers.

What are the criteria for becoming a maintainer?
What's the process?
Under what circumstances should a maintainer be removed?
What's the process there?

There are definitely further things to discuss, about maintainer diversity on the company and other levels, how big we want the maintainer pool to get, and so on. Those feel like Steering-level issues though, so once we've agreed on the process here, I'll raise another issue and we can discuss it at Steering, and get recommendations from there.