Update upgrade guide

UPGRADE.md

@@ -1,5 +1,8 @@
 ### 2.0.0
 
+- Added support for official ElasticSearch client. To avoid name collision, 
+  old `ElasticSearchHandler` renamed to `ElasticaHandler`
+
 - The timezone is now set per Logger instance and not statically, either
   via ->setTimezone or passed in the constructor. Calls to Logger::setTimezone
   should be converted.