README.md: reformatting, mention distribution packages
README.md: reformatting, mention distribution packages

* reformat so that it fits into 72 characters per line
* consistently use Gtk+
* reword some parts
* mention how to properly enable it in Debian
* mention packaged version in Debian unstable
* reference screenshots from repository, not external site

PCMan/gtk3-nocsd,PCMan/gtk3-nocsd