Could you add the license field to package.json

[hnakamur]

First of all, thanks for a great software!

Could you add the license field to package.json?
license field in package.json | npm Documentation

The following page may help for choosing a license.
Choose an open source license - Choose a License

I hope you choose a permissive license like BSD, MIT, or Apache.
Thanks!

I agree. It says it's open source in the readme but lacks a license to specify the type.