README conflates cluster and a replication group

[nk9]

The README talks about clusters a bunch, and then introduces the code which creates the Redis instance with this:

We then create the aws_elasticache_cluster resource stanza which creates our cluster.

But it then proceeds to create an aws_elasticache_replication_group, which is not the same thing as a cluster. Furthermore, there is no indication of how one would choose between creating a standalone cluster and a replication group with cluster mode either enabled or not. And since the security setup for the two resources doesn't seem to be the same (subnet_group_name vs security_group_ids), it's not easy to use the provided example for a smaller (and probably more common) setup.

It would be really helpful if the README were updated to provide more info on how to make a smaller Redis instance work. And to clarify the terminology for the given example as well.

[nk9]

This is an example of code which actually creates a cluster and not a replication group.