Clarify the license

[Nakochi-crypto]

Hi, thank you for the nice work!

This repository has no license file. package.json indicates that this software is under the MIT license, but since its body is missing, I'm not sure if it has any legal meaning.

Can you put the license file in the repository?

[Nakochi-crypto]

@liangping Thank you for adding the license file. I would have been happy if you had told me that....

By the way, package.json is still MIT. You must update it.