[Cin] BD subtitles <> dvd subtitles

[Andrea paz]

Indeed the subtitles section is lacking; but I don't have the
knowledge to improve it. Could you (Andrew, Terje or whoever uses
subtitles) write the section in more detail even with step-by-step
guides and references to external programs (and tsmuxer and mp4box)?
Then Phyllis and I will take it to Latex. You can write as much as you
like, with no length limit.