Add Cassandra to tutorial Compose set-up

Basic instructions and a Compose file for Cassandra should be added to https://github.com/debezium/debezium-examples/tree/master/tutorial.

Eventually, we'd want to have a variant of the tutorial for Cassandra, as its set-up is quite a bit different from the other (KC-based) connectors. But I'd wait with that, until the tutorial itself has been de-composed and be prepared to be published in DB-specific variants.