Add LICENSE file

[sormy]

Could you please add LICENSE file explicitly stating what license is?

[torch2424]

@sormy So I happened to come across this issue, thinking I forgot to check the license, but sometimes on JS projects it will state in the package.json instead 😄

https://github.com/calvinmetcalf/rollup-plugin-node-globals/blob/master/package.json#L20

So this project should be MIT, unless the author states otherwise on this issue 👍