Add notification in README (#845)

Add the text "“This repository is no longer actively maintained or developed.” to
the README for this public repo. UITools is the last remaining client that uses
this code & it now embeds the source in its repo.

README.md

@@ -1,2 +1,4 @@
 ### Build Status
 [![Build Status](https://dev.azure.com/xamarin/public/_apis/build/status/xamarin/Xamarin.PropertyEditing/Xamarin.PropertyEditing%20(Public)?branchName=main)](https://dev.azure.com/xamarin/public/_build/latest?definitionId=51&branchName=main)
+
+This repository is no longer actively maintained or developed.