Configure Avro serialization automatically when detecting link to a schema registry

It'd probably be nice if our Docker images and tutorial could use the Confluent Schema Registry. In fact, it'd be awesome if this were automatically configured when the Connect service is started with a link to the Schema Registry container, or do the current behavior if no such link is specified.

Update: the Avro converter has already been added to our image and is described in the documention. So we're re-purposing this issue for a usability improvement. Change the start-up script so that the Avro serializer is automatically used when detecting a link to the schema registry image (SCHEMA_REGISTRY env var is present).