clarify subcommand help method

openappstack/__main__.py

@@ -45,7 +45,9 @@ def main():  # pylint: disable=too-many-statements,too-many-branches,too-many-lo
     parser = argparse.ArgumentParser(
         prog=name,
         description='Run bootstrap script '
-        'to deploy Openappstack to a given node.')
+        'to deploy Openappstack to a given node. Note that you always need to '
+        'provide a cluster name. For subcommand specific help, for example, '
+        'run "python3 -m openappstack your-cluster install --help"')
 
     parser.add_argument(
         'cluster_name',