Update README to reference Xfce's own Solarized config

As [Xfce added their own Solarized configuration](http://git.xfce.org/apps/xfce4-terminal/commit/colorschemes/solarized-light.theme.in?id=0aab08157c72b4b76587cf89f8d4f1f235f2da24) in 2013, the README should be updated to recommend users with a recent version of Xfce try to use the supplied Solarized configuration instead of that in this repository. Should help make life :rainbow: coloured for all.
