[JOSS review] installation instructions for macOS and Windows

[justinsalamon]

The installation instructions in the README and the docs assume the user is using a linux OS that supports apt install, which isn't the case for windows and macOS.

Please add instructions for installing non-python dependencies on macOS and windows, unless this package only supports linux, in which case it should be clearly stated in the READM/docs.

I have an example of instructions for installing ffmpeg across all 3 OSs here:
https://github.com/justinsalamon/scaper/#non-python-dependencies