Add some small guide on compare page

[jesus2099]

it’s difficult to understand what we are supposed to do in compare pages without any background quick overview explanation (cf.IRC).

[lalinsky]

Any specific suggestion?

[jesus2099]

Maybe show that first column is fingerprints #19987683, second column is #25570260, that we can move #19987683 up and down and that third column is a combination of first two columns where white means different audio and black means same audio.
Something like that. :)
edit : It would also say what is the offset unit, for instance does -50 mean -50 seconds, milliseconds, frames, etc.