Add a license file

[sorawee]

GitHub's Linguist tool can't use this syntax highlighting grammar because there's no license file.

[SethPoulsen]

As far as I'm concerned it should just be licensed the same as https://github.com/brownplt/atom-language-pyret because it was taken from there. It doesn't look like that has a license file either. Any idea what it should be licensed as?

[SethPoulsen]

Following some more links around, it looks like an old version of that had MIT on it. I'm happy for this to be that too.