Add Linux package links for mkcert from repology.org

There was only a reference to Arch Linux in README.md although many other Linux distributions have a "official" package for mkcert. I took this list from repology.org: https://repology.org/project/mkcert/versions For some distributions I couldn't find a generic package link so then I linked to the latest version of that distribution. These links require maintenance, currently they are working but I'm some will break in the coming years. Maybe README.md should just link to repology.org? I don't think it's fair to only have a link to Arch Linux.