Cassandra catalog doc

This PR contains basic instructions for the Cassandra connector.

Why did I create these sections?  The documentation for connectors seems too distributed for Presto users.  As an end-user I'd like to be able to see integrated documentation in a single place.  I would also like a quick introduction to how data types, tables, and (in this case) a keyspace map to Presto schemas.

Eventually a section like this will contain tips for cassandra queries, troubleshooting, etc.

This PR builds upon - https://github.com/facebook/presto/pull/1517