Update CHANGELOG.md to include #4789

CHANGELOG.md

@@ -14,6 +14,7 @@ IMPROVEMENTS:
 * sql: updating to API version `2017-03-01-preview` [GH-4242]
 * `azurerm_app_service_slot` - support for `auto_swap_slot_name` [GH-4752]
 * `azurerm_kubernetes_cluster` - support for `enable_node_public_ip` in `agent_pool_profile` [GH-4613]
+* `azurerm_servicebus_subscription` - support for the `forward_dead_lettered_messages_to` property [GH-4789]
 * `azurerm_signalr_service` - support for the `cors` and `features` blocks [GH-4716]
 * `azurerm_sql_server` - support for the `identity` block [GH-4754]
 * `azurerm_template_deployment` - validating the ARM Template prior to deploying it, which provides more granular errors [GH-4715]