FR: better documentation for non-systemd systems, about the rootless service?

### What is the problem you're trying to solve

I am running gentoo with s6, and my user service tree is managed by, well, s6; the `containerd-rootless-setuptool.sh install` (also `check`) fails because I don't have systemd;

### Describe the solution you'd like

I would like to have a least a template to make a user service of; like a plain text file of the rootless.service file, or simply a shell script, that then can be adopted by s6, runit, openrc, and other init systems; I'll take the systemd unit service, after all it won't be that hard to convert into a shell script right?
...
right?

### Additional context

I think this would help nerdctl to run in every single type of system; maybe even windows' svc?