Add Licence

@m.hock , @manuela.richter please approve

I changed the licence to Apache 2.0 because it is more popular and less restrictive

Permissive licenses live up to their name by allowing users to do just about anything they want with the code. Users don’t have to republish any changes they make and typically only have to give attribution to the original authors in the form of a comment within the code.

whereas

Copyleft licenses require that anyone who changes your code for their software also has to release their code. For example, if you use an open source library to build your software, you’ll need to open up your code for others to operate under the same license.

Permissive licenses include the MIT license, Apache License 2.0, and the BSD License (the most popular variant of which is the BSD 3 Clause License). Copyleft licenses include the GPL license (GPL v2 and GPL v3), the AGPL License, the LGPL License, and the Mozilla Public License 2.0.
https://fossa.com/blog/how-choose-right-open-source-license/

moreover, Apache 2.0 is compatible with GNU Licences