Add folder structure information

Summary:
Some tools, like `react-native-cli` only works correctly if the `android` and `iOS` projects respect the react native folders structure. Usually, when you are integrating react native with other existing projects, that is not the case. This note will help the developers to identify and avoid potential issues.
Closes facebook#10480

Differential Revision: D4056046

Pulled By: JoelMarcey

fbshipit-source-id: ef8d75b338616b2319f7e0418a7bfcfa1a0604dc

Author: SandroMachado

docs/IntegrationWithExistingApps.md

@@ -83,12 +83,16 @@ The keys to integrating React Native components into your Android application ar
 
 The [Android Getting Started guide](/react-native/docs/getting-started.html) will install the appropriate prerequisites (e.g., `npm`) for React Native on the Android target platform and your chosen development environment.
 
+> To ensure a smooth experience, make sure your `android` project is under `$root/android`.
+
 <block class="objc swift" />
 
 ### General
 
 First, follow the [Getting Started guide](/react-native/docs/getting-started.html) for your development environment and the iOS target platform to install the prerequisites for React Native.
 
+> To ensure a smooth experience, make sure your `iOS` project is under `$root/ios`.
+
 ### CocoaPods
 
 [CocoaPods](http://cocoapods.org) is a package management tool for iOS and Mac development. We use it to add the actual React Native framework code locally into your current project.