PySide2 does not work on Windows

Should maybe be mentioned in the README.

See: https://stackoverflow.com/questions/44274151/pyside2-on-windows

```
(venv) C:\Users\X\Documents\Python Scripts\pyside>pip install PySide2
ERROR: Could not find a version that satisfies the requirement PySide2 (from versions: none)
ERROR: No matching distribution found for PySide2
```