Add documentation for JDBC url pass-through properties

As discussed with rhauch here: https://gitter.im/debezium/user?at=598c73a42723db8d5e907662, the information that database.$property gets passed to the JDBC url, is missing on the docs at: http://debezium.io/docs/connectors/mysql/#connector-properties

The PR to this ticket intends to change that.