Add upgrade notes to README

Fixes elastic#5817

README.md

@@ -10,9 +10,16 @@ Kibana is an open source ([Apache Licensed](https://github.com/elastic/kibana/bl
 ## Installation
 
 * Download: [http://www.elastic.co/downloads/kibana](http://www.elastic.co/downloads/kibana)
+* Extract the files
 * Run `bin/kibana` on unix, or `bin\kibana.bat` on Windows.
 * Visit [http://localhost:5601](http://localhost:5601)
 
+
+## Upgrade from previous version
+
+* Move any custom configurations in your old kibana.yml to your new one
+* Start or restart Kibana
+
 ## Quick Start
 
 You're up and running! Fantastic! Kibana is now running on port 5601, so point your browser at http://YOURDOMAIN.com:5601.