Update CHANGELOG.md

Author: sp1r

Diff for: CHANGELOG.md

@@ -11,6 +11,7 @@
 - [#2555](https://github.com/influxdata/kapacitor/pull/2555): run flux tasks with built-in flux engine
 - [#2559](https://github.com/influxdata/kapacitor/pull/2559): kapacitor cli supports flux tasks
 - [#2560](https://github.com/influxdata/kapacitor/pull/2560): enable new-style slack apps
+- [#2584](https://github.com/influxdata/kapacitor/pull/2584): Add custom severities to Alerta handler
 
 ### Bugfixes
 - [#2564](https://github.com/influxdata/kapacitor/pull/2564): Fix a panic in the scraper handler when debug mode is enabled