add command `docker stack services STACKNAME`

Signed-off-by: allencloud <allen.sun@daocloud.io>

experimental/docker-stacks-and-bundles.md

@@ -93,6 +93,7 @@ Commands:
   config      Print the stack configuration
   deploy      Create and update a stack
   rm          Remove the stack
+  services    List the services in the stack
   tasks       List the tasks in the stack
 
 Run 'docker stack COMMAND --help' for more information on a command.