Updated changelog with the new panel menu, Closes #770

CHANGELOG.md

 # 1.9.0 (unreleased)
+
+**UI Improvements*
+- [Issue #770](https://github.com/grafana/grafana/issues/770). UI: Panel dropdown menu replaced with a new panel menu
+
 - [Issue #877](https://github.com/grafana/grafana/issues/877). Graph: Smart auto decimal precision when using scaled unit formats
 - [Issue #850](https://github.com/grafana/grafana/issues/850). Graph: Shared tooltip that shows multiple series & crosshair line, thx @toni-moreno