Pathway from contributor to maintainer

I think this has been brought up from time to time, but I cannot seem to find an official issue for it. If you find one, we can close this (or that other one) as duplicate.

There is a need to formally document our expectations so a contributor is aware how one can go from being a contributor with no write access to a maintainer with write access. We can start with `astropy` and then downstream can adopt/adapt as needed.

I think this is a pre-cursor for:

* https://github.com/astropy/astropy-project/issues/158

Review issue #158 and search this repository for existing contributor, maintainer, or write-access policies before proposing a formal pathway. Done means the Astropy contributor-to-maintainer expectations are documented and agreed clearly enough for downstream projects to adopt or adapt.

Written by the indexing model from the issue text.