Add Mano as a maintainer

/cc @aanand @manomarks

Signed-off-by: Ben Firshman <ben@firshman.co.uk>

MAINTAINERS

@@ -1,4 +1,5 @@
 Aanand Prasad <aanand@docker.com>
 Ben Firshman <bfirsh@docker.com>
 Fernando Mayo <fernanodo.mayo@docker.com>
+Mano Marks <mano.marks@docker.com>
 Maxime Heckel <maxime.heckel@docker.com>