Update README.md

README.md

 this.props.navigator.push({
   screen: 'example.ScreenThree', // unique ID registered with Navigation.registerScreen
   title: undefined, // navigation bar title of the pushed screen (optional)
-  titleImage: undefined, //navigation bar title image instead of the title text of the pushed screen (optional)
+  titleImage: require('../../img/my_image.png'), //navigation bar title image instead of the title text of the pushed screen (optional)
   passProps: {}, // simple serializable object that will pass as props to the pushed screen (optional)
   animated: true, // does the push have transition animation or does it happen immediately (optional)
   backButtonTitle: undefined, // override the back button title (optional)