You might want to check out Notification Bar in the app store.
I don't know of a widget which gives a pop-up. But you can model this out yourself really simple. Create an entity ‘ReleaseNote’ with text attribute and microflow to create a another entity, called something like ‘ReadNote’ with a boolean, per (active) Account. Associate the ReadNote object with the ReleaseNote. Then in a microflow after signin determine if there are any ReadNote to show for the user and present the popup (after the default home target of course).
Since you mentioned as you want to notify app users, after successful User login create one landing page and top of the page or bottom bar of the page, add one bell icon for notifications. Handle this bell icon action in JS snippet. And also add the DataCounter add on, to reflect the count based on xpath on notify icon. So as notification count gets updated, when user click on notification counts, on click of each info on the notify icon, showcase a page with respect to release notes information.