Add upgrade guide entry

mmv1/third_party/terraform/website/docs/guides/version_6_upgrade.html.markdown

@@ -165,3 +165,15 @@ resource blocks that contain both fields in a conflicting pair, and remove one o
 The fields that are removed from the configuration will still have Computed values,
 that are derived from the API.
 
+## Resource: `google_compute_backend_service`
+
+### `iap.enabled` is now required in the `iap` block
+
+To apply the IAP settings to the backend service, `true` needs to be set for `enabled` field.
+
+## Resource: `google_compute_region_backend_service`
+
+### `iap.enabled` is now required in the `iap` block
+
+To apply the IAP settings to the backend service, `true` needs to be set for `enabled` field.
+