Update README.md

README.md

 ## Installation - iOS
 
  * In your project folder run `npm install react-native-navigation --save`
+> Note: We recommend using npm ver 3+. If you insist on using npm ver 2 please notice that the location for react-native-controllers in node_modules will be under the react-native-navigation folder and you'll need to change the paths in Xcode below accordingly.
  
  * Add the native files of the dependency [react-native-controllers](https://github.com/wix/react-native-controllers) to your Xcode project: