Update POSTINSTALL.md

Author: harinij

firestore-user-document/POSTINSTALL.md

@@ -11,6 +11,14 @@ When triggered by an HTTP request, this extension responds with the following sp
 
 To learn more about HTTP functions, visit the [functions documentation](https://firebase.google.com/docs/functions/http-events).
 
+# Manage the users on spreadsheet UI
+
+Once this extension is installed, you will have your users collection on Firestore. You can now manage this collection on a [Rowy](https://www.rowy.io/)'s Table UI easily with an instant admin panel. You can also invite your team members with granular access control and build any operational process such as managing subscriptions, action deactivation or deletion, building signup email flows etc. 
+
+📺 Watch the [step-by-step guided video](https://www.youtube.com/watch?v=S14fdSps9Mc) on how to view and manage your user document collection (or any Firestore data) on Rowy's spreadsheet UI.
+
+[![image](https://user-images.githubusercontent.com/307298/229514160-fa4fbf8d-c4fb-4179-98ed-7194a1612658.png)](https://demo.rowy.io/table/userManagement#sideDrawer=%22table-information%22)
+
 # Monitoring
 
 As a best practice, you can [monitor the activity](https://firebase.google.com/docs/extensions/manage-installed-extensions#monitor) of your installed extension, including checks on its health, usage, and logs.