add README.md

README.md

+# react-native-idle
+Prevent screen to sleep in ReactNative app.
+
+## Install
+
+```
+npm install react-native-idle --save
+react-native link react-native-idle
+```
+
+
+## Usage 
+
+```
+import RNIdle from 'react-native-idle'
+
+RNIdle.disableIdleTimer()
+RNIdle.enableIdleTimer()
+```