[Request] Add master.json steps to README

[L1ghtmann]

Was unfamiliar with how to obtain this and ended up following the concise instructions in https://github.com/AbCthings/vimeo-audio-video/blob/master/README.md. I imagine such instructions would be helpful for others too.

Thanks for this tool!

[chiefy]

Just a note - it looks like it's now called playlist.json not master.json