Document usage better in readme

I propose to updated the readme to make these use cases clearer:

- how to load a schema embedded in the app?
- how to load a schema via a (public) url?
- based on a swift model that is compliant with (en)Codable, take this model, serialize is to json, and validate it to one of the above schemas

(I'm staring at the code now for 15min trying to figure this out based on the test cases....)
