Add kafka-devstack-plugin to plugin-registry.rst

kafka-devstack-plugin is available now:

  Iceb054f0dfab2fff1920cc7dc36a4be256c6f161

Change-Id: I518bd079296a110a018f77b9b107883f3933e38a
diff --git a/doc/source/plugin-registry.rst b/doc/source/plugin-registry.rst
index 136a9e4..b467123 100644
--- a/doc/source/plugin-registry.rst
+++ b/doc/source/plugin-registry.rst
@@ -60,6 +60,8 @@
 +----------------------------+-------------------------------------------------------------------------+
 |devstack-plugin-hdfs        |git://git.openstack.org/openstack/devstack-plugin-hdfs                   |
 +----------------------------+-------------------------------------------------------------------------+
+|devstack-plugin-kafka       |git://git.openstack.org/openstack/devstack-plugin-kafka                  |
++----------------------------+-------------------------------------------------------------------------+
 |devstack-plugin-pika        |git://git.openstack.org/openstack/devstack-plugin-pika                   |
 +----------------------------+-------------------------------------------------------------------------+
 |devstack-plugin-sheepdog    |git://git.openstack.org/openstack/devstack-plugin-sheepdog               |