MAINTAINERS: add mailing list for man-pages

Nowadays, man-pages has an associated mailing list.  Mention that list
in MAINTAINERS.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>