Add upstream quote about elastic 5.x vm.max_map_count check

Author: yosifkit

elasticsearch/content.md

@@ -21,6 +21,10 @@ See the following sections of the upstream documentation for more information:
 -	[Setup Elasticsearch » Important System Configuration » Virtual memory](https://www.elastic.co/guide/en/elasticsearch/reference/5.0/vm-max-map-count.html)
 -	[Setup Elasticsearch » Bootstrap Checks » Maximum map count check](https://www.elastic.co/guide/en/elasticsearch/reference/5.0/_maximum_map_count_check.html)
 
+This [comment in elastic/elasticsearch#4978](https://github.com/elastic/elasticsearch/issues/4978#issuecomment-258676104) shows why this change was added in upstream. Production mode is when listening on non-localhost.
+
+> Elasticsearch will not start in production mode if `vm.max_map_count` is not high enough. [...] If the value on your system is NOT high enough, then your cluster is going to crash and burn at some stage and you will lose data.
+
 ## Running Containers
 
 You can run the default `elasticsearch` command simply: