[Gluster-Maintainers] Updating Maintainers Guide

[Kaushal M]

Hello maintainers!

If you didn't know, we have a maintainers guide at [1]. This document
describes what is expected from a GlusterFS maintainer, and is a quick
guide on maintainer-ship.

But this document is very light at the moment, and can be improved.
So, I'd like to know what other information can be added to the doc.
I'll start.

- Properly describe the responsibilities of different types of
maintainers; sub-maintainers, release-maintainers etc.
- Define the process for becoming a maintainer (Neils came up with
this actually)

Please go through the document and reply with your opinions.

Thanks,
Kaushal

[1] https://gluster.readthedocs.org/en/latest/Contributors-Guide/Guidelines-For-Maintainers/