add more detailed licensing information

Debian is actually really accurate about the licensing information of upstream packages (which I regard as a feature), so a non-existing verbatim copyright line is [one of the reasons](https://lintian.debian.org/tags/copyright-without-copyright-notice.html) a package will be [rejected](https://ftp-master.debian.org/REJECT-FAQ.html). To fix this, I have added the GPL v2 in the file `LICENSE`, and extended `README.md` with a short copyright line, followed by the example text from GPL's section "How to Apply These Terms to Your New Programs". This should do it :-) 

Of course, as the upstream author, feel free to change everything to your needs, especially the Copyright line – I have taken my version from the nordlicht man page.