add package.json file

[Sinewyk]

You don't have to publish on npm to at least allow npm users to directly install from github (even if it's better to be on npm). But that can be done later.

example dependency after this is merged:

"mangopay-cardregistration-js-kit": "Mangopay/cardregistration-js-kit#4e0110eca964c3437bdaceacf50acdda274ca920",

[hobailey]

Thanks @Sinewyk!