Update README.md

[leotm]

See

• https://github.com/react-native-community/react-native-template-typescript/blob/master/template/ios/HelloWorld/AppDelegate.m
• https://github.com/flatlogic/react-native-starter/blob/master/ios/ReactNativeStarter/AppDelegate.m
• https://github.com/BranchMetrics/react-native-branch-deep-linking-attribution/tree/master/examples
• etc.

[kyleclegg]

This change updates documentation from Swift to Objective-C. We are ok with the documentation remaining in Swift.