Document graphics driver extensions

[orowith2os]

Closes https://github.com/flatpak/flatpak-docs/issues/381

I plan to base this off of @TingPing's article over at https://blog.tingping.se/2018/08/26/flatpak-host-extensions.html, and make some use of the mesa-git flatpak extension: https://gitlab.com/freedesktop-sdk/mesa-git-extension

This is a pretty barebones MR right now, so it might be nonfunctional; I'll sort it out later.

[orowith2os]

First manually signed commit! Hooray!