replace "clone in user namespace" with instructions for how to run rpmoci without root using unshare

[tofay]

Per feedback in #147.

The rootless support in #46 should be achievable with unshare --map-auto --map-root-user --user*, so long rpmoci selects dnf cache/log directories that the user has permission to write to.

*rpmoci is mostly used on mariner 2.0, whose unshare doesn't have --map-auto and --map-root-user, but we'll be upgrading soon to an OS that does have those options.