Document the removal of Simple* and that the KafkaClient is the new one, not the old one

[jeffwidman]

This is a followup for #1196.

Need to document that the SimpleClient/SimpleConsumer/SimpleProducer are all gone and that if you need them, use an older version.

Also need to document that KafkaClient now refers to the new async client, not the old deprecated one.

Honestly, the changelog is probably the only place that really needs it... maybe at the bottom of the readme and the top of the KafkaClient docs could mention the change as well for folks confused on why KafkaClient behavior suddenly changed.

[jeffwidman]

This was covered in the changelog: ed8c9cb