document freeSlideBehind option

[jobevers]

I'm not sure what freeSlideBehind is for: https://github.com/gre/diaporama/blob/80e977ce03ac194fc8b9ce149b809f43a0bef522/lib/Diaporama.js#L210

[gre]

this allows to (optionally) remove slides that are freeSlideBehind times before the current slide so the memory doesn't grow indefinitely for stream purpose.

An example is running here: http://greweb.me/diaporama/#example/4 , if you accelerate the speed of the diaporama (or manually click on Next), you will see that the number of slides never reach 40 slides, because previous slides are released (code is here https://github.com/gre/diaporama-website/blob/master/stream_example.js ).

Note that diaporama is designed as a "timeline", so the way to make an infinite stream of images is to append some images at the end, and remove slides at the beginning (+ also changing the time) while the time is running.

[jobevers]

Thanks!