Clarify license

[dbohdan]

The readme currently says, "All configuration files included are public domain. Do what you want with them.", but also "Copyleft 2012-2023 Matthias Saou". Is the word "copyleft" just a stylistic choice, or are parts of this repository under a copyleft license?

P.S.: Thanks for this project.

[anarcat]

we are considering packaging this software in Debian, and such clarification would be extremely welcome.

Both options ("public domain" and "copyleft") are generally allowed in Debian but the latter is unclear enough that it might cause trouble in our legal auditing processes.

Thanks!