Add documentation

[edmorley]

Whilst the Python CNB already has pretty comprehensive user-facing build log output and error messages, plus lots of developer-facing code comments, we also need:

• User-facing usage instructions in the readme (such as how to use with pack build)
• Description of features + differences compared to the classic Heroku Python buildpack
• Developer facing workflow docs (eg how to develop locally, run tests, publish a new buildpack version etc)

Internal tracking epic