Add more info to README.

Author: ehsannas

firestore/testapp/README.md

@@ -34,6 +34,8 @@ Building and Running the testapp
         ```
 
     -   Update the pod versions in the Podfile to match the C++ SDK version that you are using.
+        For the latest version of the C++ SDK, you can find the associated pod versions on the
+        [`Add Firebase to your project` page](https://firebase.google.com/docs/cpp/setup?platform=ios#libraries-ios).
         For instance: if you're using SDK version 8.2.0, use the following in the Podfile:
 
         ```