Show the driver docs in the README

Its not at all obvious from this repo what keys/values entries in `platforms` can take. Having just done some digging to find out why my platforms didn't have networking, I finally found the docstring in [src/module_podman/driver.py]() which made it obvious that the molecule driver needed `network:` not `networks:` like the docker driver I'd been using.

It's a good markdown-format docstring, be good to expose it or at least link to it from the README.

Start with the existing documentation in src/module_podman/driver.py and the repository README. Expose or link the driver docstring from the README so the valid platforms keys and values, including the networking key, are easy to find; confirm the resulting Markdown renders correctly.

Written by the indexing model from the issue text.