docs(changelog): adds note about fixing avg() reducer bug

CHANGELOG.md

@@ -21,6 +21,7 @@
 
 ### Bugfixes
 * **Alerting**: Add alert message to webhook notifications [#6807](https://github.com/grafana/grafana/issues/6807)
+* **Alerting**: Fixes a bug where avg() reducer treated null as zero. [6c9cf87](https://github.com/grafana/grafana/commit/6c9cf87080e52966846a48d04209d90c166ca42e)
 * **PNG Rendering**: Fix for server side rendering when using non default http addr bind and domain setting [#6813](https://github.com/grafana/grafana/issues/6813)
 * **PNG Rendering**: Fix for server side rendering when setting enforce_domain to true [#6769](https://github.com/grafana/grafana/issues/6769)
 * **Webhooks**: Add content type json to outgoing webhooks [#6822](https://github.com/grafana/grafana/issues/6822)