Please add license text(s)

[krasimirgg]

The license field in Cargo.toml indicates that this package is licensed under MIT. It's good to have the license(s) text(s) in the repository as well (and into the cargo package) so this work can be properly attributed downstream.

Example from another crate that's licensed under MIT:
https://github.com/davidbarsky/tracing-tree/blob/main/LICENSE-MIT