diff --git a/plugin/clouddns/README.md b/plugin/clouddns/README.md
index 6ff4a2b24..1e122813f 100644
--- a/plugin/clouddns/README.md
+++ b/plugin/clouddns/README.md
@@ -34,6 +34,9 @@ clouddns [ZONE:PROJECT_ID:HOSTED_ZONE_NAME...] {
     accessed.
 
 *   `credentials` is used for reading the credential file from **FILENAME** (normally a .json file).
+    This field is optional. If this field is not provided then authentication will be done automatically,
+    e.g., through environmental variable `GOOGLE_APPLICATION_CREDENTIALS`. Please see
+    Google Cloud's [authentication method](https://cloud.google.com/docs/authentication) for more details.
 
 *   `fallthrough` If zone matches and no record can be generated, pass request to the next plugin.
     If **[ZONES...]** is omitted, then fallthrough happens for all zones for which the plugin is