Specify when something is considered F/OSS software.

This adds links to [the GNU/FSF definition of Free Software](https://www.gnu.org/philosophy/free-sw.html) and its [compatible licences](https://www.gnu.org/licenses/license-list.html) to the README file to specify what is meant with F/OSS software.

It also allows other licences as long as they have been reviewed by the Open Source Initiative. This is mainly added as a back-up and I think this way it will cover all current projects.

Change sparked by the debate in #883.