Martin Bengtsson has put together a cool script that you can use to develop your own toast notifications to warn users about available upgrades or PC reboots.
The Windows 10 Toast Notification Script enables you to create nice and nifty toast notifications for the logged on user in Windows 10. This can be done with Configuration Manager or with Group Policy Preferences and Scheduled Tasks (and probably other means too).
Download: Windows 10 Toast Notification Script
Configuration and usage information: Windows 10 Toast Notification Script