Add epilog to parser

Other clients have an epilogue at the end of the parser
that shows up when the help flag is used. This adds the
epilogue to aid in using the barbican client.
Documentation is updated as well.

Change-Id: I0463aa5c980e8337fb06e70c083763fddb2c0746

README.rst

@@ -149,6 +149,8 @@ The command line client is self-documenting. Use the --help flag to access the u
       --os-key <key>        Defaults to env[OS_KEY].
       --timeout <seconds>   Set request timeout (in seconds).
 
+    See "barbican help COMMAND" for help on a specific command.
+
     Commands:
       complete       print bash completion command
       container create  Store a container in Barbican.

barbicanclient/barbican.py

@@ -285,6 +285,8 @@ class Barbican(app.App):
                             metavar='<barbican-api-version>',
                             default=client.env('BARBICAN_API_VERSION'),
                             help='Defaults to env[BARBICAN_API_VERSION].')
+        parser.epilog = ('See "barbican help COMMAND" for help '
+                         'on a specific command.')
         session.Session.register_cli_options(parser)
         return parser