Add License

[jeyemwey]

I'd like to add grafana-to-ntfy to the NixOS package repository nixpkgs.

I kindly request you to add a suitable open-source license to the project. A well-defined license will not only ensure the legal use and distribution of the software but also facilitate its inclusion in various package management systems, including nixpkgs.

There a numerous resources such as choosealicense.com to help choosing a license, but the default is sadly unfree.

Thanks!