Docs: Add a maintenance mode info to README

Closes jquerygh-2150

Author: mgol

README.md

@@ -1,5 +1,7 @@
 # [jQuery UI](http://jqueryui.com/) - Interactions and Widgets for the web
 
+_**Note:** jQuery UI is in maintenance-only mode. Please read [the project status blog post](https://blog.jqueryui.com/2021/10/jquery-maintainers-update-and-transition-jquery-ui-as-part-of-overall-modernization-efforts/) for more information._
+
 jQuery UI is a curated set of user interface interactions, effects, widgets, and themes built on top of jQuery. Whether you're building highly interactive web applications, or you just need to add a date picker to a form control, jQuery UI is the perfect choice.
 
 If you want to use jQuery UI, go to [jqueryui.com](http://jqueryui.com) to get started, [jqueryui.com/demos/](http://jqueryui.com/demos/) for demos, [api.jqueryui.com](http://api.jqueryui.com/) for API documentation, or the [Using jQuery UI Forum](http://forum.jquery.com/using-jquery-ui) for discussions and questions.