MAINTAINERS: Add link to drm-misc documentation

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Acked-by: Jani Nikula <jani.nikula@linux.intel.com>
Acked-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161123143132.29006-1-daniel.vetter@ffwll.ch