README: description, contributing

Set up the usual project rules, following what wayland-protocols and wayland use.

Granting permissions is eased given this is mostly for documentation.

This assumes that !1 (merged) will land providing the license file.

Cc @swick, @vitalyp, @NaveenKumar, @hwentland - please make sure to subscribe to Issues and MRs in this project so I can stop mentioning you all explicitly.