Improve documentation of available features and options

[lerebear]

The configuration docs in the README have started to lag behind the functionality provided by this action. Although the JSON schema does have a canonical catalog of all available options, users of this action don't ordinarily have a reason to look at it, and I doubt many would.

We should provide a complete, human-readable guide to all the configuration options provided by this action. One approach to doing this might be to write formal documentation in the JSON schema, and then serialize to a more readable format (e.g. YAML, so we have a catalog similar to what's provided in the README of actions/checkout).