Update README.md

Author: rdelrosario

README.md

@@ -5,6 +5,7 @@
   - [**Apple Pay/GooglePay**](http://www.xamboy.com/2020/02/11/braintree-integration-in-xamarin-forms-part-2/)
   - [**PayPal**](http://www.xamboy.com/2020/02/28/braintree-integration-in-xamarin-forms-part-3/)
   - [**Drop-in UI**](http://www.xamboy.com/2020/03/24/braintree-integration-in-xamarin-forms-part-4/)
+  - [**Braintree Upgrading**](https://www.xamboy.com/2021/04/01/updating-braintree-to-the-latest-version-in-xamarin-forms/)
 
 <p align="center">
 <img width="300" height:"700" src="dropUi.gif" title="Android"/>