chromium: Add myself (@primeos) as provisional maintainer

I don't really have the hardware resources nor time to do this properly,
but I'll try to keep a watch on Chromium (updates, PRs, and issues)
until we've found a new team [0].
Testing will be performed on a best effort basis (no guarantees :o).

I've also briefly documented the current maintainer
roles/responsibilities and added `meta.longDescription`.

[0]: https://github.com/NixOS/nixpkgs/issues/78450