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>

CONTRIBUTING

@@ -0,0 +1,105 @@
+Contributing to libdrm
+======================
+
+Submitting Patches
+------------------
+
+Patches should be sent to dri-devel@lists.freedesktop.org, using git
+send-email. For patches only touching driver specific code one of the driver
+mailing lists (like amd-gfx@lists.freedesktop.org) is also appropriate. See git
+documentation for help:
+
+http://git-scm.com/documentation
+
+Since dri-devel is a very busy mailing list please use --subject-prefix="PATCH
+libdrm" to make it easier to find libdrm patches. This is best done by running
+
+    git config --local format.subjectprefix "PATCH libdrm"
+
+The first line of a commit message should contain a prefix indicating what part
+is affected by the patch followed by one sentence that describes the change. For
+examples:
+
+    amdgpu: Use uint32_t i in amdgpu_find_bo_by_cpu_mapping
+
+The body of the commit message should describe what the patch changes and why,
+and also note any particular side effects. For a recommended reading on
+writing commit messages, see:
+
+http://who-t.blogspot.de/2009/12/on-commit-messages.html
+
+Your patches should also include a Signed-off-by line with your name and email
+address. If you're not the patch's original author, you should also gather
+S-o-b's by them (and/or whomever gave the patch to you.) The significance of
+this is that it certifies that you created the patch, that it was created under
+an appropriate open source license, or provided to you under those terms.  This
+lets us indicate a chain of responsibility for the copyright status of the code.
+For more details:
+
+https://developercertificate.org/
+
+We won't reject patches that lack S-o-b, but it is strongly recommended.
+
+Review and Merging
+------------------
+
+Patches should have at least one positive review (Reviewed-by: tag) or
+indication of approval (Acked-by: tag) before merging. For any code shared
+between drivers this is mandatory.
+
+Please note that kernel/userspace API header files have special rules, see
+include/drm/README.
+
+Coding style in the project loosely follows the CodingStyle of the linux kernel:
+
+https://www.kernel.org/doc/html/latest/process/coding-style.html?highlight=coding%20style
+
+Commit Rights
+-------------
+
+Commit rights will be granted to anyone who requests them and fulfills the
+below criteria:
+
+- Submitted a few (5-10 as a rule of thumb) non-trivial (not just simple
+  spelling fixes and whitespace adjustment) patches that have been merged
+  already. Since libdrm is just a glue library between the kernel and userspace
+  drivers, merged patches to those components also count towards the commit
+  criteria.
+
+- Are actively participating on discussions about their work (on the mailing
+  list or IRC). This should not be interpreted as a requirement to review other
+  peoples patches but just make sure that patch submission isn't one-way
+  communication. Cross-review is still highly encouraged.
+
+- Will be regularly contributing further patches. This includes regular
+  contributors to other parts of the open source graphics stack who only
+  do the oddball rare patch within libdrm itself.
+
+- Agrees to use their commit rights in accordance with the documented merge
+  criteria, tools, and processes.
+
+To apply for commit rights ("Developer" role in gitlab) send a mail to
+dri-devel@lists.freedesktop.org and please ping the maintainers if your request
+is stuck.
+
+Committers are encouraged to request their commit rights get removed when they
+no longer contribute to the project. Commit rights will be reinstated when they
+come back to the project.
+
+Maintainers and committers should encourage contributors to request commit
+rights, as especially junior contributors tend to underestimate their skills.
+
+Code of Conduct
+---------------
+
+Please be aware the fd.o Code of Conduct also applies to libdrm:
+
+https://www.freedesktop.org/wiki/CodeOfConduct/
+
+See the gitlab project owners for contact details of the libdrm maintainers.
+
+Abuse of commit rights, like engaging in commit fights or willfully pushing
+patches that violate the documented merge criteria, will also be handled through
+the Code of Conduct enforcement process.
+
+Happy hacking!