Minor update for documentation to include PySide6

[petercolemanfoundry]

Currently in the README.md it says "Qt.py enables you to write software that runs on any of the 4 supported bindings - PySide2, PyQt5, PySide and PyQt4."

As Qt.py now supports for PySide6, please could this be updated? There are a couple of places which mention the 4 languages, rather than 5.

This will help make it clearer to users that Qt.py supports the latest PySide version, after great work done by the contributors.

Kind Regards

Peter

[mottosso]

PR is more than welcome!

[MHendricks]

I'm addressing this in #431