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

MAINTAINERS

@@ -4026,6 +4026,7 @@ DRM DRIVERS AND MISC GPU PATCHES
 M:	Daniel Vetter <daniel.vetter@intel.com>
 M:	Jani Nikula <jani.nikula@linux.intel.com>
 M:	Sean Paul <seanpaul@chromium.org>
+W:	https://01.org/linuxgraphics/gfx-docs/maintainer-tools/drm-misc.html
 S:	Maintained
 T:	git git://anongit.freedesktop.org/drm/drm-misc
 F:	Documentation/gpu/