MAINTAINERS: add Joe Betz

Joe has been helping fix lots of bugs and involved
in critical feature design/implementation,
plus leading patch release process for over a year.

Thanks for your excellent work.

Signed-off-by: Gyuho Lee <gyuhox@gmail.com>

MAINTAINERS

@@ -11,6 +11,7 @@ Brandon Philips <bphilips@redhat.com> (@philips) pkg:*
 Fanmin Shi <fashi@redhat.com> (@fanminshi) pkg:*
 Gyuho Lee <gyuhox@gmail.com> <gylee@redhat.com> (@gyuho) pkg:*
 Hitoshi Mitake <h.mitake@gmail.com> (@mitake) pkg:*
+Joe Betz <jpbetz@google.com> (@jpbetz) pkg:*
 Xiang Li <xiangli.cs@gmail.com> (@xiang90) pkg:*
 
 Ben Darnell <ben@cockroachlabs.com> (@bdarnell) pkg:github.com/coreos/etcd/raft