docs: add fork disclaimer in the README

Author: MightyCreak

README.md

@@ -63,3 +63,6 @@ Copyright (C) 2006-2019 Derrick Moser <[email protected]>
 Copyright (C) 2015-2023 Romain Failliot <[email protected]>
 
 Icon made by [@jimmac](https://github.com/jimmac).
+
+This repository is a fork of the original project on SourceForge, which doesn't
+seem to be maintained anymore: <https://sourceforge.net/projects/diffuse/>.