Create maintainership document

[RichiH]

This document could be part of governance, or it could stand alone. It should lay out what is expected of any maintainer and some common baselines of how to work with their own repos. For example, working through pull requests instead of pushing directly to master ensures that we always have some review and that no single-person projects exist; per definition, at least two people will need to take an interest to make PR-based workflows work.

[roidelapluie]

in VP we have it in the governance:

https://github.com/voxpupuli/plumbing/blob/master/share/governance.md