Help text nudge to read the friendly manual (#327)

* Help text nudge to read the friendly manual

* remove help text when no verb provided

* Change name of constant

* avoid alignment of wrapped lines

* save some vertical space (as it was before)

Co-authored-by: Dirk Thomas <[email protected]>

Author: rotu

colcon_core/command.py

@@ -211,8 +211,10 @@ def _parse_optional(self, arg_string):
     parser = CustomArgumentParser(
         prog=get_prog_name(),
         formatter_class=CustomFormatter,
-        epilog=get_environment_variables_epilog(
-            environment_variables_group_name))
+        epilog=(
+            get_environment_variables_epilog(
+                environment_variables_group_name
+            ) + '\n\n' + READTHEDOCS_MESSAGE))
 
     # enable introspecting and intercepting all command line arguments
     parser = decorate_argument_parser(parser)
@@ -268,6 +270,10 @@ def get_environment_variables_epilog(group_name):
     return 'Environment variables:\n' + '\n'.join(epilog_lines)
 
 
+READTHEDOCS_MESSAGE = 'For more help and usage tips, see ' \
+    'https://colcon.readthedocs.io'
+
+
 def add_log_level_argument(parser):
     """
     Add the argument for the log level to the parser.