Add What's New in Grafana 3.0 link into the readme (#5001)

README.md

@@ -16,6 +16,7 @@ Graphite, Elasticsearch, OpenTSDB, Prometheus and InfluxDB.
 - [What's New in Grafana 2.0](http://docs.grafana.org/guides/whats-new-in-v2/)
 - [What's New in Grafana 2.1](http://docs.grafana.org/guides/whats-new-in-v2-1/)
 - [What's New in Grafana 2.5](http://docs.grafana.org/guides/whats-new-in-v2-5/)
+- [What's New in Grafana 3.0](http://docs.grafana.org/guides/whats-new-in-v3/)
 
 ## Features
 ### Graphite Target Editor