Please, add a license file

[trebmuh]

Reading at the source file headers, one would assume it is a GNU-GPL v2 license, same as Musescore, but it would be better to add a license file to this repository.

[mirabilos]

Eh? The source code files do include the licence header.

[trebmuh]

@mirabilos sure, but a LICENCE file in the repo's root directory would be nice to have