message: integration/docs: Clarify GCS credentials discovery config requirement
diff: @@ -36,6 +36,13 @@ Then make sure the environment variables `GCS_CLIENT_EMAIL` and
  Since this integration uses the Google Storage SDK, you can also choose to not
  provide any explicit credentials and let the SDK discover them automatically.
  +Please note that for this method to work you must add an empty `googleGcs` object to the `integrations` section in `app-config.yaml`.
  +
  +```yaml
  +integrations:
  + googleGcs: {}
  +```
  +
  One of these discovery methods is to provide an environment variable called
  `GOOGLE_APPLICATION_CREDENTIALS` and set it to the file path of your JSON
  service account key.
  