Add basic CONTRIBUTING file

I picked up a bunch of the pieces from wayland's version:

https://gitlab.freedesktop.org/wayland/wayland/blob/master/CONTRIBUTING.md



The weston one is fairly similar. Then I rather massively trimmed it
down since in reality libdrm is a bit a dumping ground with very few
real rules. The commit rights and CoC sections I've copied verbatim
from igt respectively drm-misc. Weston/Wayland only differ in their
pick of how many patches you need (10 instead of 5). I think for
libdrm this is supremely relevant, since most everyone will get their
commit rights by contributing already to the kernel or mesa and having
commit rights there already.

Anyway, I figured this is good to get the rules documented, even if
there's mostly not many rules.

Note: This references maintainers in a MAINTAINERS file, which needs
to be created first.

Note: With the gitlab migration the entire commit rights process is
still a bit up in the air. But gitlab commit rights and roles are
hierarchical, so we can do libdrm-only maintainer/commiter roles
("Owner" and "Developer" in gitlab-speak). This should avoid
conflating libdrm roles with mesa roles, useful for those pushing to
libdrm as primarily kernel contributors.

v2: Comments from Emil:
- Recommend subject prefix.
- Fix copypaste fumbles, this isn't igt/wayland ...

v3: Comments from Marek:
- libdrm moved to mesa, update the document. Atm the entire account
  request situation is entirely not clear for gitlab and mesa
  projects, so that's a bit up in the air. Also, should probably send
  an announcement to dri-devel@, which didn't happen.
- amd folks don't submit their patches to dri-devel, document that.
  Probably applies to other drivers too.

v4: Comments from Rob:
- Also include kernel/userspace in the commit counts criteria, due to
  libdrm's special role as a glue library.

v5: Summarize the irc discussion on gitlab roles in the commit message
a bit.

v6: Some grammer stuff from Eric E.

v7: Use --local in git config (Eric E.)

Cc: Dave Airlie <airlied@gmail.com>
Cc: Michel Dänzer <michel@daenzer.net>
Cc: Emil Velikov <emil.velikov@collabora.com>
Cc: Marek Olšák <marek.olsak@amd.com>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Rob Clark <robdclark@gmail.com> (v4)
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com> (v6)
Acked-by: Emil Velikov <emil.l.velikov@gmail.com> (v6)
Acked-by: Marek Olšák <marek.olsak@amd.com> (v5)
Acked-by: Dave Airlie <airlied@redhat.com>
References: https://gitlab.freedesktop.org/wayland/weston/blob/master/CONTRIBUTING.md
References: https://01.org/linuxgraphics/gfx-docs/maintainer-tools/drm-misc.html#commit-rights
References: https://cgit.freedesktop.org/drm/igt-gpu-tools/tree/CONTRIBUTING#n54


Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>