📝 docs: simplify README footer

The appdirs fork history is no longer relevant since the original project
was deprecated in 2023. Removed the fork attribution text and replaced it
with a direct link to CONTRIBUTING.md to make it easier for new contributors
to find guidelines.

Author: gaborbernat

README.md

@@ -66,5 +66,4 @@ Full documentation is available at [platformdirs.readthedocs.io](https://platfor
 - **[Platform details](https://platformdirs.readthedocs.io/en/latest/platforms.html)** -- default paths for each
   operating system
 
-Contributions are welcome! This project is a maintained fork of the original
-[appdirs](https://github.com/ActiveState/appdirs) library.
+Contributions are welcome! See [CONTRIBUTING.md](CONTRIBUTING.md) for details.