headers: Add README file

Since we're trying to standardise and make things more consistent in
the area, add a basic README which covers some of the more popular
topics.

v2:
 - Drop drm-misc (Daniel Vetter)
v3:
 - Elaborate on when and which headers to update
 - Add a list of headers and the respective "issues"
 - Add file to EXTRA_DIST

Cc: Dave Airlie <airlied@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> (v1)
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>