changelog: adds notes for #14509 and #15179

CHANGELOG.md

 # Unreleased
 
+# 6.0.0 stable (unreleased)
+
+### Bug Fixes
+* **Stackdriver**: fix for float64 bounds for distribution metrics [#14509](https://github.com/grafana/grafana/issues/14509)
+* **Stackdriver**: no reducers available for distribution type [#15179](https://github.com/grafana/grafana/issues/15179)
+
 # 6.0.0-beta3 (2019-02-19)
 
 ### Minor