Provide example for batches and updates

[moripaku]

A couple of examples would be helpful.

qb package appears deprecated, but there is no other apparent support for batches in currently latest version (2.8.0).

As there is no evident way to extract values from gocqlx.Queryx for gocql.Batch.Query function either, it is not clear how gocql.Batch is supposed to be used with gocqlx now.

Similarly with updates, there is only Insert/Select examples, but an update example would be also appreciated.

[nigel5]

In the master branch you can bindstruct to a batch (https://github.com/scylladb/gocqlx/blob/master/batchx_test.go) but it's not released yet. I forked the repo and released it to my own private branch.