Bump min supported iOS/Android version to 11/19

Summary: Changelog: [iOS][Android] Bumped min supported iOS/Android version in README to reflect reality

Reviewed By: fkgozali

Differential Revision: D25917083

fbshipit-source-id: 2061b25a8b47f9ebedb922b85fe3edd3646338f2

README.md

@@ -67,7 +67,7 @@ React Native is developed and supported by many companies and individual core co
 
 ## 📋 Requirements
 
-React Native apps may target iOS 10.0 and Android 4.1 (API 16) or newer. You may use Windows, macOS, or Linux as your development operating system, though building and running iOS apps is limited to macOS. Tools like [Expo](https://expo.io) can be used to work around this.
+React Native apps may target iOS 11.0 and Android 5.0 (API 21) or newer. You may use Windows, macOS, or Linux as your development operating system, though building and running iOS apps is limited to macOS. Tools like [Expo](https://expo.io) can be used to work around this.
 
 ## 🎉 Building your first React Native app