Document Datastax driver version support

[akhaku]

Hi, the README indicates that this is built on datastax 3.x, but looking through Jira/some if the issues it looks like it is now based on datastax 4.x as of 3.0.0. Just to confirm, this means that datastax 3.x is incompatible?

[mp911de]

Good catch and thanks for bringing this to our attention. We need to update the docs to reflect that Spring Data Cassandra 3.x is based on Datastax driver 4.x