README: Mention how to build for MSVC and UWP
README: Mention how to build for MSVC and UWP

This was missed when we added Visual Studio support.

Part-of: <https://gitlab.freedesktop.org/gstreamer/cerbero/-/merge_requests/527>

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