create helk group ID, and document consumer threads for scaling

docker/helk-logstash/pipeline/0002-kafka-input.conf

@@ -21,5 +21,10 @@ input {
     max_poll_interval_ms => "300000"
     #############################
     max_poll_records => "500"
+    # If you have multiple logstash instances, this is your ID so that each instance consumes a slice of the Kafka pie.
+    # No need to change this unless you know what your doing and for some reason have the need
+    group_id => "helk_logstash"
+    # Change to number of Kafka partitions, only change/set if scaling on large environment & customized your Kafka paritions
+    #consumer_threads => 1
   }
 }