Mention COPR package

[dngray]

This package was originally part of secure-boot in which the maintainer mentioned:

[2023-11-12 Update]: This repo is deprecated because I have switched to using systemd-ukify for generating UKIs. Because sbctl is very useful on its own, I will continue to package it in a separate repo. Folks who currently use this repo should expect things to continue to work, though no further updates will be provided and repos for Fedora 40+ will not be created.

I asked the maintainer if he'd mind if we added this to the official readme via email. Reply was:

Sure, I don't mind at all. I actively use sbctl on all my machines, so I intend to keep maintaining the package for the foreseeable future (unless Fedora adds it to the official repos).

[Foxboron]

Thanks!