Added a note about index naming convention during upgrade

Author: axelaris

README.md

@@ -11,11 +11,17 @@ Logsearch < v23.0.0 was based on Elasticsearch 1.x and Kibana 3.
 
 Logsearch > v200 is based on Elasticsearch 2.x and Kibana 4.
 
-There is NO upgrade path from Elasticsearch 1.x to 2.x.  Sorry :(
+- There is NO upgrade path from Elasticsearch 1.x to 2.x.  Sorry :(
 
-Logsearch > v204.0.0 is based on Elasticsearch 5.x and Kibana 5.
+Logsearch > v204.0.0 is based on Elastic stack version 5.
+
+- For upgrade procedure from Elasticsearch 2.x please refer to [v205.0.0 release notes](https://github.com/cloudfoundry-community/logsearch-boshrelease/releases/tag/v205.0.0#component-updates).
+
+Logsearch > v210.0.0 is based on Elastic stack version 6.
+
+- Elasticsearch 6.x can use indices created in Elasticsearch 5.x, but not those created in Elasticsearch 2.x or before.
+- **Important**: After upgrading running 5.x cluster to 6.x all existing indicies will be available for reading data. However, writing to these indicies is not possible. In order to write data immediatelly after upgrade you have to [change index naming convention](https://github.com/cloudfoundry-community/logsearch-boshrelease/commit/2f83b41ee14dbe3141e21cc0c40df340d50e0169). As long as index names are usually based on current date, this change can be safely reverted in a day or so.
 
-For upgrade procedure from Elasticsearch 2.x please refer to [v205.0.0 release notes](https://github.com/cloudfoundry-community/logsearch-boshrelease/releases/tag/v205.0.0#component-updates).
 
 ## Getting Started