[enhancement] Add MIT license also to npm per package.json

[sthagen]

When projects using this nice package want to document the license, it is easier when on npm the license shown at https://www.npmjs.com/package/react-rotary-knob-skin-pack is not none but matches the one of the source repository (MIT).

In case this helps, I am happy to provide a pull request.
But, maybe reviewing some persons' changes of your package.json is more work, than changing it yourself ( in case you want to do so, of course).

Thank you @hugozap for sharing this package with the world, much appreciated.