Add license disclaimer to README
PropEr is using GPL-3.0-or-later license for their codebase which
requires property testing files to be licensed as GPL. Fortunately that
doesn't affect main codebase, which allows licensing them independently.
That is enormously confusing combo IMHO, but there currently no
workaround, as PropEr team do not want to change license even when it is
IMHO causing confusion.