Add license info to Cargo manifests

Hi,
Thank you for creating PET; it's a nifty project.
It would be great to have [license](https://doc.rust-lang.org/cargo/reference/manifest.html#the-license-and-license-file-fields) fields filled out in manifests for packages, as that would make it easier to use with projects that use `cargo about`; as is, users have to polyfill PET license info in their own about.toml files.