Updated changelog with merged PR #850, shared multi series graph tooltip & crosshair

CHANGELOG.md

 # 1.9.0 (unreleased)
 - [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
 
 # 1.8.1 (unreleased)