Update Installing.md (#3870)

Some extra help concerning a common issue that occurs with fresh install of Xcode + react native 0.56. https://github.com/react-community/create-react-native-app/issues/316#issuecomment-351371080

docs/docs/Installing.md

 	@end
 	```
 
+3a. If, in Xcode, you see the following error message in `AppDelegate.m` next to `#import "RCTBundleURLProvider.h": 
+```
+! 'RCTBundleURLProvider.h' file not found
+```
+This is because the `React` scheme is missing from your project. You can verify this by opening the `Product` menu and the `Scheme` submenu. 
+
+To make the `React` scheme available to your project, run `npm install -g react-native-git-upgrade` followed by `react-native-git-upgrade`. Once this is done, you can click back to the menu in Xcode: `Product -> Scheme -> Manage Schemes`, then click '+' to add a new scheme. From the `Target` menu, select "React", and click the checkbox to make the scheme `shared`. This should make the error disappear.
+
 ## Android
 
 > Make sure your Android Studio installation is updated. We recommend editing `gradle` and `java` files in Android Studio as the IDE will suggest fixes and point out errors, this way you avoid most common pitfalls.