Added note on max # of docs allowed in a shard

docs/reference/getting-started.asciidoc

@@ -89,6 +89,8 @@ The number of shards and replicas can be defined per index at the time the index
 
 By default, each index in Elasticsearch is allocated 5 primary shards and 1 replica which means that if you have at least two nodes in your cluster, your index will have 5 primary shards and another 5 replica shards (1 complete replica) for a total of 10 shards per index.
 
+NOTE: An Elasticsearch shard is a Lucene index.  There is a maximum number of documents you can have in a single Lucene index.  As of https://issues.apache.org/jira/browse/LUCENE-5843[`LUCENE-5843`], the limit is `2,147,483,519` (= Integer.MAX_VALUE - 128) documents. 
+
 With that out of the way, let's get started with the fun part...
 
 == Installation