Add filecache docs

[bochi]

add brief documentaiton on the filecache feature

[tbrisker]

@sbernhard can you please review?

[sbernhard]

Thanks @bochi
I think we should put this to a v 13.1 or even 14.0

[bochi]

Thanks @bochi
I think we should put this to a v 13.1 or even 14.0

Ok, so - 13.1 or 14? ;-)

[bochi]

closing for #1653