Add license

[ghost]

People cannot use or modify your project without violating copyright laws due to the lack of a license. Please add an open-source license to this project.

See: https://help.github.com/articles/open-source-licensing/#what-happens-if-i-dont-choose-a-license

[FenrirWolf]

The file (which is pretty much copypasta'd from rust-lang/rust) states it's dual-licensed under Apache/MIT. Is that not sufficient?