add comment.

Author: zhumin8

oauth2_http/java/com/google/auth/oauth2/UserCredentials.java

@@ -124,6 +124,7 @@ static UserCredentials fromJson(Map<String, Object> json, HttpTransportFactory t
           "Error reading user credential from JSON, "
               + " expecting 'client_id', 'client_secret' and 'refresh_token'.");
     }
+    // currently "token_uri" is not a default field and needs to be added to json file manually
     String tokenUrl = (String) json.get("token_uri");
     URI tokenUri = (tokenUrl == null || tokenUrl.isEmpty()) ? null : URI.create(tokenUrl);
     return UserCredentials.newBuilder()