MAINTAINERS: document and add "Q" patchwork queue entries

Patchwork queues show the acceptance/rejection state of submitted patches
for various MAINTAINER trees.  Document their existence.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>