docs(app, ios): note that AppDelegate.mm or .m is fine

Related #6625

Author: mikehardy

docs/index.md

@@ -100,7 +100,7 @@ Select the downloaded `GoogleService-Info.plist` file from your computer, and en
 
 To allow Firebase on iOS to use the credentials, the Firebase iOS SDK must be configured during the bootstrap phase of your application.
 
-To do this, open your `/ios/{projectName}/AppDelegate.m` file, and add the following:
+To do this, open your `/ios/{projectName}/AppDelegate.mm` file (or `AppDelegate.m` if on older react-native), and add the following:
 
 At the top of the file, import the Firebase SDK: