community/podman: add message in post-install script about rootless podman

Discussed this with minimal in IRC. podman is supposed to be a daemonless program, so users will often not expect to enable the openrc init service. However, there are some things in the openrc service that are necessary for rootless podman to run, and that might not be obvious. Warn users about them. This comes from my own experience trying to setup an unified cgroup hierarchy, where I modified rc_group_mode in /etc/rc.conf, to see how openrc ignored it until I enabled the cgroups service.

Marked as Draft waiting for maintainer comments/approval