MAINTAINERS: add sheepdog development mailing list

This will help people find mailing list relevant to sheepdog.

Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

MAINTAINERS

@@ -879,6 +879,7 @@ F: block/rbd.c
 Sheepdog
 M: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
 M: Liu Yuan <namei.unix@gmail.com>
+L: sheepdog@lists.wpkg.org
 S: Supported
 F: block/sheepdog.c