Deprecate iOS 9 in readme (facebook#27670)

Summary:
## Changelog

[CATEGORY] [TYPE] - Message
Pull Request resolved: facebook#27670

Differential Revision: D19296782

Pulled By: cpojer

fbshipit-source-id: 7a0b5fc1e6bfa5b4b34ceeeefe41e1f902278a0c

README.md

@@ -70,7 +70,7 @@ React Native is developed and supported by many companies and individual core co
 
 ## 📋 Requirements
 
-React Native apps may target iOS 9.0 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 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.
 
 ## 🎉 Building your first React Native app