README: add some instructions for building on Windows
README: add some instructions for building on Windows

1. directive to install.NET 3.5.1, needed by Wix
2. directive to delete fake Windows Store Python in path (Windows 10 only)

Part-of: <https://gitlab.freedesktop.org/gstreamer/cerbero/-/merge_requests/581>

nirbheek/cerbero,nirbheek/cerbero,GStreamer/cerbero,nirbheek/cerbero,GStreamer/cerbero,GStreamer/cerbero,GStreamer/cerbero,nirbheek/cerbero,GStreamer/cerbero