update CHANGELOG.md to include #6275

Author: katbyte

CHANGELOG.md

@@ -4,6 +4,10 @@
 IMPROVEMENTS:
 
 * dependencies: updating `github.com/Azure/azure-sdk-for-go` to `v41.2.0` [GH-6419]
+* Data Source: `azurerm_policy_definition`: can now lookup with `name` [GH-6275]
+* Data Source: `azurerm_policy_definition`: the `management_group_id` has been deprecated and renamed to `management_group_name` [GH-6275]
+* `azurerm_policy_definition`: the `management_group_id` has been deprecated and renamed to `management_group_name` [GH-6275]
+
 
 ## 2.5.0 (April 09, 2020)