add note about fallback authentication for route53 provider

octodns/provider/route53.py

@@ -217,11 +217,14 @@ class Route53Provider(BaseProvider):
 
     route53:
         class: octodns.provider.route53.Route53Provider
-        # The AWS access key id (required)
+        # The AWS access key id
         access_key_id:
-        # The AWS secret access key (required)
+        # The AWS secret access key
         secret_access_key:
 
+    Alternatively, you may leave out access_key_id and secret_access_key,
+    this will result in boto3 deciding authentication dynamically.
+
     In general the account used will need full permissions on Route53.
     '''
     SUPPORTS_GEO = True