Update README.md

Author: aksalj

README.md

@@ -18,7 +18,9 @@ $ composer require africastalking/africastalking
 
 ## Usage
 
-The SDK needs to be instantiated using your username and API key (which you can get from the [dashboard](https://account/africastalking.com)). 
+The SDK needs to be instantiated using your username and API key, which you can get from the [dashboard](https://account/africastalking.com).
+
+> You can use this SDK for either production or sandbox apps. For sandbox, the app username is **ALWAYS** `sandbox`
 
 ```php
 <?php