Put a file CACHEDIR.TAG when creating a cache dir

[cmarqu]

Hi,

the following idea (redux: "avoid backup of cache dirs") seems nice,
and maybe hishel should support it: https://bford.info/cachedir/

Basically, add a file CACHEDIR.TAG with the following content:

Signature: 8a477f597d28d172789f06886806bc55
# This file is a cache directory tag created by hishel.
# For information about cache directory tags, see:
#	http://www.brynosaurus.com/cachedir/