[Request] Update Google Setup for iOS in README

[vuphuctho]

Current guide for deeplink setup in iOS for Google is not informative enough, not to mention the link https://dev.twitter.com/cards/mobile/url-schemes is not helpful at all.

My suggest is to add location of where to put the deeplink info in iOS in your example: https://github.com/adamjmcgrath/ReactNativeSimpleAuthExample/blob/master/ios/ReactNativeSimpleAuthExample/Info.plist#L38-L41, similar to the Android guide

Or we can put guide of how to add deeplink using XCode, e.g. like in this Medium's post:

1. Open info.plist and at the top of the file, create a new property called URL types

2. Expand item 0 (zero) and choose URL Schemes.

3. Give item 0 the name you would like your app to be linkable as. In our case, I chose peopleapp as the name.