Update design-decisions.md

docs/internals/design-decisions.md

@@ -12,3 +12,6 @@ the core developers.
    messages should be translated when these are displayed to non-tech end user and make sense to him. HTTP status messages,
    exceptions about the code etc. should not be translated. Console messages are always in English because of encoding
    and codepage handling difficulties.
+3. **[Adding new auth client support](https://github.com/yiisoft/yii2/issues/1652)**
+   For better maintenability, we will not add any additional auth clients to the core extension. They should be done 
+   in terms of user extensions.