Add docs to show how to select specific simulator.

Summary:
Add a message to let people know they can use the `--simulator` flag to run their apps on different simulators instead of the default "iPhone 6"
Closes facebook#8078

Differential Revision: D3464912

Pulled By: JoelMarcey

fbshipit-source-id: b59d5061d2b3501618602932fcc285bac99b7573

docs/RunningOnDeviceIOS.md

@@ -4,7 +4,7 @@ title: Running On Device
 layout: docs
 category: Guides (iOS)
 permalink: docs/running-on-device-ios.html
-next: communication-ios
+next: running-on-simulator-ios
 ---
 
 Note that running on device requires [Apple Developer account](https://developer.apple.com/register) and provisioning your iPhone. This guide covers only React Native specific topic.

docs/RunningOnSimulatorIOS.md

@@ -0,0 +1,18 @@
+---
+id: running-on-simulator-ios
+title: Running On Simulator
+layout: docs
+category: Guides (iOS)
+permalink: docs/running-on-simulator-ios.html
+next: communication-ios
+---
+
+## Starting the simulator
+
+Once you have your React Native project initialized, you can run `react-native run-ios` inside the newly created project directory. If everything is set up correctly, you should see your new app running in the iOS Simulator shortly.
+
+## Specifying a device
+
+You can specify the device the simulator should run with the `--simulator` flag, followed by the device name as a string. The default is `"iPhone 6"`. If you wish to run your app on an iPhone 4s, just run `react-native run-ios --simulator "iPhone 4s"`.
+
+The device names correspond to the list of devices available in Xcode. You can check your available devices by running `xcrun simctl list devices` from the console.