[Docs] Add supported iOS version to getting started

docs/GettingStarted.md

@@ -9,7 +9,7 @@ next: tutorial
 
 ## Requirements
 
-1. OS X - This repo only contains the iOS implementation right now, and Xcode only runs on Mac.
+1. OS X - This repo only contains the iOS (7+) implementation right now, and Xcode only runs on Mac.
 2. New to Xcode?  [Download it](https://developer.apple.com/xcode/downloads/) from the Mac App Store.
 3. [Homebrew](http://brew.sh/) is the recommended way to install node, watchman, and flow.
 4. `brew install node`. New to [node](https://nodejs.org/) or [npm](https://docs.npmjs.com/)?