Update README.md

Author: stanleycyang

README.md

@@ -398,6 +398,10 @@ Delete app / Restart device / Quit "store" related processes in Activity Monitor
 #### getAvailablePurchases return empty array.
 - `getAvailablePurchases` is used only when you purchase a non-consumable product. This can be restored only. If you want to find out if a user subscribes the product, you should check the receipt which you should store in your own database. Apple suggests you handle this in your own backend to do things like what you are trying to achieve.
 
+#### Using Face ID & Touch to checkout on iOS
+
+- After you have completed the setup and set your deployment target to iOS 12, FaceID and Touch to purchase will be activated by default in production. Please note that in development or TestFlight, it will **NOT** use FaceID / Touch to checkout because they are using the Sandbox environment.
+
 
 ## Supporting react-native-iap