Add documentation on ffmpeg location

[boazharel2]

I have been trying to get this to work for a while now and I keep coming up against the following error:

yt_dlp.utils.DownloadError: ERROR: Postprocessing: ffprobe and ffmpeg not found. Please install or provide the path using --ffmpeg-location

I have ffmpeg installed per the readme, but I found nothing that explains how to specify the location so that album-splitter can find it. May this is something we can add?