Add manila to devstack plugin registry

Manila has had a devstack plugin since Kilo.  The registry should
reflect that.

Change-Id: I910198495e98b6f8d92c4880d1bee0c16d6c1559
diff --git a/doc/source/plugin-registry.rst b/doc/source/plugin-registry.rst
index 1b54c01..bbab56b 100644
--- a/doc/source/plugin-registry.rst
+++ b/doc/source/plugin-registry.rst
@@ -26,6 +26,8 @@
 +--------------------+-------------------------------------------+--------------------+
 |magnum              |git://git.openstack.org/openstack/magnum   |                    |
 +--------------------+-------------------------------------------+--------------------+
+|manila              |git://git.openstack.org/openstack/manila   | file shares        |
++--------------------+-------------------------------------------+--------------------+
 |mistral             |git://git.openstack.org/openstack/mistral  |                    |
 +--------------------+-------------------------------------------+--------------------+
 |rally               |git://git.openstack.org/openstack/rally    |                    |