📄 LICENSE: Add CC BY-SA 4.0

This commit explicitly adds a LICENSE.txt file to the repository. This is the default content license for the Fedora Project (per the Fedora Project Contributor Agreement, or FPCA). The license is being added to bring us into compliance with GitLab's Open Source program, which requires that licensing information is provided in the repository.

For lack of any other license, this means that until now, content in this repo (including issues) is technically under CC BY-SA 4.0. This is making it explicit.