Add information about verifying Xcode Command Line Tools are installed

Summary:
<img width="694" alt="screen shot 2016-05-19 at 10 12 35 am" src="https://cloud.githubusercontent.com/assets/3757713/15402657/3f9a5896-1daa-11e6-842d-7a6c57b86b9f.png">
Closes #7637

Differential Revision: D3332705

Pulled By: JoelMarcey

fbshipit-source-id: a01b98305fe1b11559e7c9893e7844cb37599fc4

docs/QuickStart-GettingStarted.md

@@ -97,7 +97,9 @@ npm install -g react-native-cli
 
 #### Xcode
 
-[Xcode](https://developer.apple.com/xcode/downloads/) 7.0 or higher. Open the App Store or go to https://developer.apple.com/xcode/downloads/. This will also install `git` as well.
+[Xcode](https://developer.apple.com/xcode/downloads/) 7.0 or higher is required. You can install Xcode via the App Store or [Apple developer downloads](https://developer.apple.com/xcode/downloads/). This will install the Xcode IDE and Xcode Command Line Tools.
+
+> While generally installed by default, you can verify that the Xcode Command Line Tools are installed by launching Xcode and selecting `Xcode | Preferences | Locations` and ensuring there is a version of the command line tools shown in the `Command Line Tools` list box. The Command Line Tools give you `git`, etc.
 
 <block class="mac android" />