drm: Document code of conduct

freedesktop.org has adopted a formal&enforced code of conduct:

https://www.fooishbar.org/blog/fdo-contributor-covenant/
https://www.freedesktop.org/wiki/CodeOfConduct/



Besides formalizing things a bit more I don't think this changes
anything for us, we've already peer-enforced respectful and
constructive interactions since a long time. But it's good to document
things properly.

v2: Drop confusing note from commit message and clarify the grammer
(Chris, Alex and others).

Cc: Daniel Stone <daniels@collabora.com>
Cc: Keith Packard <keithp@keithp.com>
Cc: tfheen@err.no
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Sumit Semwal <sumit.semwal@linaro.org>
Acked-by: Archit Taneja <architt@codeaurora.org>
Reviewed-by: Martin Peres <martin.peres@free.fr>
Acked-by: Thierry Reding <treding@nvidia.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Vincent Abriou <vincent.abriou@st.com>
Acked-by: Neil Arm...