Bump iOS min in Readme (facebook#20805)

Summary:
iOS 8 was dropped as of 0.56.
Pull Request resolved: facebook#20805

Differential Revision: D9481968

Pulled By: hramos

fbshipit-source-id: 23e74ac3145ba92e2c119be365f2435e290c3f61

Author: hramos

README.md

@@ -11,7 +11,7 @@ The focus of React Native is on developer efficiency across all the platforms yo
 
 See the official [React Native website](https://facebook.github.io/react-native/) for an introduction to React Native.
 
-Supported operating systems are >= Android 4.1 (API 16) and >= iOS 8.0.
+Supported operating systems are >= Android 4.1 (API 16) and >= iOS 9.0.
 
 - [Getting Started](#getting-started)
 - [Documentation](#documentation)