Add LGPL v2.1 license file

[qmfrederik]

Based on the license headers in the individual source code files, it appears this project uses LGPL 2.1 or later. Add a COPYING file.

[fredkiefer]

Could you please update the address of the FSF? Otherwise this looks fine.

[qmfrederik]

@fredkiefer Turns out that the latest version of the LGPL-2.1 license as published at https://www.gnu.org/licenses/old-licenses/lgpl-2.1.txt no longer includes their address but just a link to https://fsf.org/ .

Interestingly, in "How to Apply These Terms to Your New Libraries", they also changed the (fake) name from Ty Coon to Moe Ghoul.

I've included a verbatim copy of the latest version from https://www.gnu.org/licenses/old-licenses/lgpl-2.1.txt, let me know if that works for you.