Add LICENSE file

[jsdanielh]

Add LICENSE file to state MIT as the license of this repository.
This closes #33.

[oxarbitrage]

I think MIT should be ok here as this project was made from scratch however i am not totally sure, we better call @dconnolly or @mpguerra to figure out this.

[teor2345]

Can you also add Apache-2.0 LICENSE file?

You can see an example of how we do licenses in the Zebra repository:

• https://github.com/ZcashFoundation/zebra

And here's the current license for this repository, as a SPDX identifier:

ed25519-zebra/Cargo.toml

Line 8 in d351240

license = "MIT OR Apache-2.0"

[jsdanielh]

Can you also add Apache-2.0 LICENSE file?

You can see an example of how we do licenses in the Zebra repository:

* https://github.com/ZcashFoundation/zebra

And here's the current license for this repository, as a SPDX identifier:

ed25519-zebra/Cargo.toml

Line 8 in d351240

license = "MIT OR Apache-2.0"

Done. Also added a small description of the license in the README.md file.

[teor2345]

Thanks, this looks good!

I'll merge it now, so it gets in the upcoming release.
(cc @dconnolly)