docs(readme): update description

Author: richzw

README.md

@@ -1,7 +1,8 @@
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-[App Store Server API](https://developer.apple.com/documentation/appstoreserverapi) Golang Client
+Apple App Store Server Golang Library
 ================
 
+The Golang server library for the [App Store Server API](https://developer.apple.com/documentation/appstoreserverapi) and [App Store Server Notifications](https://developer.apple.com/documentation/appstoreservernotifications).
+
 The App Store Server API is a REST API that you call from your server to request and provide information about your customers' in-app purchases. 
 
 The App Store Server API is independent of the app’s installation status on the customers’ devices. The App Store server returns information based on a customer’s in-app purchase history regardless of whether the customer installs, removes, or reinstalls the app on their devices.