add warning to sonar.properties about ES Virtual Memory on Linux

sonar-application/src/main/assembly/conf/sonar.properties

@@ -186,6 +186,17 @@
 # ELASTICSEARCH
 # Elasticsearch is used to facilitate fast and accurate information retrieval.
 # It is executed in a dedicated Java process. Default heap size is 512Mb.
+#
+# --------------------------------------------------
+# Word of caution for Linux users on 64bits systems
+# --------------------------------------------------
+# Please ensure Virtual Memory on your system is correctly configured for Elasticsearch to run properly
+# (see https://www.elastic.co/guide/en/elasticsearch/reference/5.5/vm-max-map-count.html for details).
+#
+# When SonarQube runs standalone, a warning such as the following may appear in logs/es.log:
+#      "max virtual memory areas vm.max_map_count [65530] is too low, increase to at least [262144]"
+# When SonarQube runs as a cluster, however, Elasticsearch will refuse to start.
+#
 
 # JVM options of Elasticsearch process
 #sonar.search.javaOpts=-Xms512m \