doc: Split out README.rst

Again this is just to make things look neater on gitlab. For the
curious, I pushed this series to

https://gitlab.freedesktop.org/drm/maintainer-tools/tree/master



Long-term we might want o polish this more, especially in case we
decide to embrace gitlab merge requests and issues for
maintainer-tools. But this should be good enough for now.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>