CONTRIBUTING.md: Add section on becoming a maintainer

[Mindavi]

I got some feedback after a review on a PR (#161404 (comment)) that it's not very clear that adding yourself as a maintainer in maintainers.nix should be a separate commit.

It might be nice to add a small section to CONTRIBUTING explaining the (simple) process, or maybe linking from there to the manual.

[fabaff]

The Nixpkgs Manual has that section.

[VuiMuich]

Nevertheless the section in the manual is helpful, imo having a comment in CONTRIBUTING seems like a good idea.

Additionally I would suggest that both sources should mention that as per comment it might be preferrable to have the commits ordered like this:

maintainers: add <new-maintainer>
<package-name>: <commit message>

this also helps to easier squash required changes to the actual commit.

[fricklerhandwerk]

Fixed by #248013 and #250344