Add info about native google sign in

README.md

@@ -57,6 +57,10 @@ You're running the sample code on the UI thread. Strict mode policy will throw a
 
 This library is meant to be a Java implementation of the API. Google Volley is specific to Android and should not be introduced in this library. However, if you still want to refactor it, you should create it as a separate project.
 
+   - How can I use Android's native Google sign in with this libary?
+
+You can't. The Google Indentity Platform uses the SHA1 fingerprint and package name to authenticate the caller of all sign in requests. This means that Niantic would need to add your app's SHA1 fingerprint and package name to their Google API Console. If you ever requested a Google Maps API key, you went through the same process. An alternative would be using a WebView to access the web based OAuth flow. This will work with the client ID and secret provided by this library.
+
 
 ## Contributing
   - Fork it!