docs: provide recommendation for strategy of managing multiple target images

[kgilmer]

Expected behavior

Documentation of how one might use the samge OS/version image for multiple VMs on a single host.

Actual behavior

After reading the README and the wiki docs, I'm still unsure of the best* way of managing multiple images of the same OS/version on my computer. I <3 quickemu as a tool for desktop env testing, and tend to keep multiple images of a given OS release for various tasks, such as testing at each stage (unstable, testing, release), and sometimes some specific topic. I notice the config file allows specifying a path to the qemu disk image, and if that file does not exist, it creates one. That seems like the most obvious way to do this, but am generally unsure if there is any shared state or other footguns in the os image directories that may interfere with each other.

*best = in my case, best is the simplest, most obvious, and least footgun-ey approach