add changelog entry

Author: aallawala

CHANGELOG.md

@@ -67,6 +67,7 @@
 * [ENHANCEMENT] Add recording rules to improve responsiveness of Alertmanager dashboard. #387
 * [ENHANCEMENT] Add `CortexRolloutStuck` alert. #405
 * [ENHANCEMENT] Added `CortexKVStoreFailure` alert. #406
+* [ENHANCEMENT] Add ability to override `datasource` for generated dashboards. #407
 * [BUGFIX] Fixed `CortexIngesterHasNotShippedBlocks` alert false positive in case an ingester instance had ingested samples in the past, then no traffic was received for a long period and then it started receiving samples again. #308
 * [BUGFIX] Alertmanager: fixed `--alertmanager.cluster.peers` CLI flag passed to alertmanager when HA is enabled. #329
 * [BUGFIX] Fixed `CortexInconsistentRuntimeConfig` metric. #335