Add Xcode version to dependencies in README

Author: morganchen12

README.md

@@ -10,7 +10,7 @@ FirebaseUI clients are also available for [Android](https://github.com/firebase/
 
 ## Installing FirebaseUI for iOS
 
-FirebaseUI supports iOS 8.0+. We recommend using [CocoaPods](https://cocoapods.org/pods/FirebaseUI), add
+FirebaseUI supports iOS 8.0+ and Xcode 11+. We recommend using [CocoaPods](https://cocoapods.org/pods/FirebaseUI), add
 the following to your `Podfile`:
 
 ```ruby