Add a GitHub parseable license file

[kmturley]

Hello
Thank-you for open-sourcing your instrument!

Currently you have a custom license.txt file:
https://github.com/musescore/mdl/blob/master/license.txt

GitHub cannot parse custom licenses in the UI

Or provide them via the search API:

"license": {
    "key": "other",
    "name": "Other",
},

https://api.github.com/repos/musescore/mdl

Using a license template will fix this and make your repo searchable by license:
https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/licensing-a-repository

Thanks!