Consider adding a license to this project

[LanHikari22]

Hi,

It's not clear to me if this is unlicensed work. Please consider adding an explicit LICENSE file or similar in the project.

Also consider updating the README with a heading for the license.

If this work is meant to be unlicensed, you can also explicitly state so in the README.

I do notice in crate.io github-slugger that you use the apache-2.0 license, so this could be added here also.

Thanks!
Mohammed