docs: media: document media multi-committers rules and process

docs: media: document media multi-committers rules and process

As the media subsystem will experiment with a multi-committers model,
update the Maintainer's entry profile to the new rules, and add a file
documenting the process to become a committer and to maintain such
rights.

Reviewed-by: Ricardo Ribalda <ribalda@chromium.org>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>