Add plug-in summary for osc doc

Stevedore Sphinx extension handles this comment.
http://docs.openstack.org/developer/python-openstackclient/
plugin-commands.html

Change-Id: I7e7f4f8714c0a261d587d1382103f4f367b64248

neutronclient/osc/plugin.py

@@ -11,6 +11,8 @@
 #   under the License.
 #
 
+"""OpenStackClient plugin for advanced Networking service."""
+
 import logging
 
 # TODO(rtheis/amotoki): Add functional test infrastructure for OSC