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 Armstrong <narmstrong@baylibre.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Brian Starkey <brian.starkey@arm.com>
Acked-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Acked-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Acked-by: Michel Dänzer <michel.daenzer@amd.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
Acked-by: Keith Packard <keithp@keithp.com>
Acked-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Acked-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>