Add GitLab contact information, suggest pinging authors

When you want to change something, it's useful to know who to ping to get things reviewed. I've found a bunch of MRs today where the author really should've been pinged and done something about it, so let submitters know how to do it.

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I'd suggest everyone on this list (@Zamundaaa @afrantzis @andri @apol @carlosg @daniels @davidre @emersion @frog @gfxstrand @i509VCB @ifreund @jadahl @jpetersen @kennylevinsen @linkmauve @mak @mvlad @ofourdan @pq @whot @Zamundaaa @zmike) consider if the entries are still appropriate, and add/remove people as appropriate via other MRs.