Documentation & error message unclear when trying to generate for urllib

[drsybren]

Describe the bug
The README.md describes that urllib is a library that's supported by this generator. There is no documentation on how to do this, though. Using --library urllib gives this error:

Usage: openapi-python-generator [OPTIONS] SOURCE OUTPUT
Try 'openapi-python-generator --help' for help.

Error: Invalid value for '--library': urllib

Update: reading through the README again, it's clear to me that urllib is not yet supported. Still, I feel that either --help or --library <invalid value> should provide a list of valid/supported/usable values.

To Reproduce

1. Run openapi-python-generator --library urllib a b
2. See the error message

Expected behavior
I expected a list of valid values for the --library argument. And a mention in the documentation of supported values.

Additional context
This is with openapi-python-generator, version 0.5.0.