please add a license (vaild SPDX identifier) to your package.json

[Oliver Bleckmann]

Public Domain is no vaild identifier PD licenses are:
"CC0-1.0", "PDDL-1.0", "SAX-PD" or "Unlicense"
see https://spdx.org/licenses/ for more

[John Beech]

Please add license to this project.