Updated README.md to fix #230

Thanks @p5k6!
adding archive command + XDG_* environnment variables (not finished)

In its essence, the archive command does this :
  archive /tmp/TEST.png
  mv /tmp/2013-10-20--TEST.png

I follow here the principle from gmail that you should 98% of the time,
archiving is better of deleting.

XDG Base Directory Specification
http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html

Please review this patch, I'm not a real bash programmer.