doc: write down rules we have for dependencies

Mesa is a core component in many distributions, so it feels like we really need at least something written down.

I hope I didn't write down anything controversial and allowed for enough loopholes to get stuff in as long as it's something experimental and not considered to be a default by any means or in special cases like driconf.

To make it more explicit: This should describe something we already agree on and shouldn't sneak in anything new we didn't enforce before. This is purely for writing it down to make it more official and for external people to know.