Add links to unofficial windows setup

[SiddharthManthan]

I have created a Windows Setup for mpv. The features of the installer are described in the project’s README.

I would like to publish it on WinGet. The package will be named mpv unofficial. This is a stopgap until official WinGet packages become available. Microsoft requires that installer URLs be discoverable from the official publisher’s website in order to be accepted, hence this pr.