README: document --hidden for dealing with . prefixed files
README: document --hidden for dealing with . prefixed files

Fixes #868

Documents `--hidden` for dealing with . prefixed files that are
hidden on *NIX like systems.

Dedsec01/drive,odeke-em/drive,gccpacman/drive,gccpacman/drive,odeke-em/drive,Dedsec01/drive