Docs: Added new elasticsearch-river-kafka plugin to the documentation (u...

...ses latest version of Kafka, EL Bulk API, and supports concurrent requests)

The differences between existing and this new plugin:

* uses Kafka 0.8.1.1 (latest version)
* uses Elasticsearch 1.4.0 (latest version)
* makes use of Bulk API (to have better performance)
* can be executed in concurrent requests
* abstracts the consuming mechanism (can consume Kafka messages from multiple brokers and partitions)

@clintongormley please review (a follow up on the closed PL - https://github.com/elasticsearch/elasticsearch/pull/8443)