docs: document how to consume messages with a message schema

[pypingou]

The current docs has examples and instructions on how to send messages using a message schema but not of consuming messages that have a schema.

How does a consumer know that a message it received follows a schema? How does it convert the base Message into a more specific object?

[jeremycline]

All the standard Python object introspection interfaces should be sufficient. The consumer is called with an instance of the class that defines the schema, if it's installed. If it's not, it'll get a plain Message. Anything from isinstance to poking around in the attached jsonschema (although if you're doing this you probably shouldn't be) should work as you'd expect. I think it's fine to add a note to the consumer docs saying something like that.