[plugin/clouddns] Add additional doc in explaining credentials file field is optional (#5231)

This PR adds additional doc in explaining credentials file field is optional,
in case user might be concerned to save an filename location in Corefile.

Technically since Corefile does not save plaintext secret for clouddns
(unlikely route53/azure), this is not too big of a concern, as far as I can see.
Still it is worth to pointing out in documentation.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

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