Add congress to plugin registry

Now congress supports devstack plugin model, hence including the
same in plugin registry

Change-Id: I1ac83c529a466e6a75a9b46ed9b56085140a63ed
diff --git a/doc/source/plugin-registry.rst b/doc/source/plugin-registry.rst
index 49b3a7f..c68d926 100644
--- a/doc/source/plugin-registry.rst
+++ b/doc/source/plugin-registry.rst
@@ -26,6 +26,8 @@
 +------------------+---------------------------------------------+--------------------+
 |ceilometer        |git://git.openstack.org/openstack/ceilometer | metering           |
 +------------------+---------------------------------------------+--------------------+
+|congress          |git://git.openstack.org/openstack/congress   | governance         |
++------------------+---------------------------------------------+--------------------+
 |gnocchi           |git://git.openstack.org/openstack/gnocchi    | metric             |
 +------------------+---------------------------------------------+--------------------+
 |magnum            |git://git.openstack.org/openstack/magnum     |                    |