Define progression path for contributor to maintainer

[waldyrious]

Collecting notes I've put elsewhere:

From #1048 (comment):

[We should] ensure resilience of the project over the long term by making the barriers between user, contributor and maintainer as permeable as possible -- case in point: none of the the currently active maintainers were part of the original team. We want this fluid adaptation to continue into the future.

From #980 (comment):

One thing we clearly need is a way to handle the natural flow of maintainers: people's availability varies over time, so a hand-picked, small maintainer team won't be resilient in the long-run. I suggest something more permeable, inspired in the model used by Wikipedia: after a contributor has, say, 3 PRs merged, they should automatically get commit rights, and after X more commits they should be made part of the tldr-pages organization. If this could be implemented automatically, even better. That way the project is able to scale naturally, and avoid the stagnation that often plagues to such endeavors once the original team loses steam.

[waldyrious]

There are interesting notes in http://docs.brew.sh/#maintainers which we could take some inspiration from. But let's try making the whole thing lighter (i.e. less documents to read, and more automated steps).

[sbrl]

I'd want to field-test the whole thing for a while first in manual-mode - to make sure we get the right balance.