Adding error message when include is part of CLI (#18)

* Including file include from the commandline

* Adding tests

* Fixing help string tests

* Adding content to README.md

* Adding error conditions

draccus/argparsing.py

@@ -143,7 +143,12 @@ def _postprocessing(self, parsed_args: Namespace) -> T:
         for key in parsed_arg_values:
             parsed_value = cfgparsing.parse_string(parsed_arg_values[key])
             if isinstance(parsed_value, str) and parsed_value.startswith("include"):
-                parsed_arg_values[key] = cfgparsing.load_config(open(parsed_value[8:], "r"))
+                try:
+                    parsed_arg_values[key] = cfgparsing.load_config(open(parsed_value[8:], "r"))
+                except FileNotFoundError as e:
+                    raise FileNotFoundError(f"{e}. Include is a reserved cli keyword. "
+                                            f"If your argument uses include, "
+                                            f"Please refer to https://github.com/dlwh/draccus/issues/17")
             else:
                 parsed_arg_values[key] = parsed_value