added reference to Java client API (see #37)

Author: spujadas

docs/index.md

@@ -70,7 +70,7 @@ This command publishes the following ports, which are needed for proper operatio
 - 5044 (Logstash Beats interface, receives logs from Beats such as Filebeat – see the *[Forwarding logs with Filebeat](#forwarding-logs-filebeat)* section).
 - 5000 (Logstash Lumberjack interface, receives logs from Logstash forwarders – see the *[Forwarding logs with Logstash forwarder](#forwarding-logs-logstash-forwarder)* section).
 
-**Note** – The image also exposes Elasticsearch's transport interface on port 9300. Use the `-p 9300:9300` option with the `docker` command above to publish it.
+**Note** – The image also exposes Elasticsearch's transport interface on port 9300. Use the `-p 9300:9300` option with the `docker` command above to publish it. This transport interface is notably used by [Elasticsearch's Java client API](https://www.elastic.co/guide/en/elasticsearch/client/java-api/current/index.html).
 
 The figure below shows how the pieces fit together.