Add 'v' for consistency (expo#252)

Added 'v' in front of 6 on line 12 to keep the document more consistent.
Line 45 specifies versions using 'v' before version numbers and 12 didn't.

Author: jae-huh

README.md

@@ -9,7 +9,7 @@ Once you're up and running with Create React Native App, visit [this tutorial](h
 
 ## Quick Overview
 
-Make sure you have Node 6 or later installed. No Xcode or Android Studio installation is required.
+Make sure you have Node v6 or later installed. No Xcode or Android Studio installation is required.
 
 ```sh
 $ npm install -g create-react-native-app