build: make it easier to force FFmpeg upstream

Apparently some people want this. Actually making it compile is still
their problem, though, and I expect that build with FFmpeg upstream will
occasionally be broken (as it is right now). This is because mpv also
relies on API provided by Libav, and if FFmpeg hasn't merged that yet,
it's not our problem - we provide a version of FFmpeg upstream with
those changes merged, and it's called ffmpeg-mpv.

Also adjust the README which still talked about FFmpeg releases.