Fix LICENSE for GitHub and other use cases

[robo9k]

Seems like #41 broke GitHub's parsing of the repo license. According to https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/adding-a-license-to-a-repository they want a file named "LICENSE or LICENSE.md (with all caps)".

The README.md already contains a "license" section, but no SPDX id. The Cargo.toml also includes a license field.

Apart from GitHub maybe some research can gather expectations from other tools, e.g. a LICENSE.spdx file seems to be used elsewhere.