Add the overlay permission information

Summary:
If your current android application is targeting the android `api level 23` or greater, displaying the error view will cause a crash in the application. The crash only shows that the system cannot display the view.
Closes facebook#10479

Differential Revision: D4167801

Pulled By: hramos

fbshipit-source-id: 5f7315038a8a4c36d1513173139d5c4cf50ea868

Author: SandroMachado

docs/IntegrationWithExistingApps.md

@@ -728,6 +728,8 @@ public boolean onKeyUp(int keyCode, KeyEvent event) {
 
 That's it, your activity is ready to run some JavaScript code.
 
+> If your app is targeting the Android `api level 23` or greater, make sure you have, for the development build, the `overlay permission` enabled. You can check it with `Settings.canDrawOverlays(this);`. This is required because, if your app produces an error in the react native component, the error view is displayed above all the other windows. Due to the new permissions system, introduced in the api level 23, the user needs to approve it.
+
 ## Run your app
 
 To run your app, you need to first start the development server. To do this, simply run the following command in your root folder: