Documentation: local dev/test workflow

[jhaynes]

What I'd like:

When building a bottlerocket image, I get some disk images before converting them into an AMI. I'd like to be able to test them locally (e.g. via qemuor VMDK images) with full connectivity.

[mdaniel]

There is already a comment saying "for local testing with qemu," so even just adding a quick doc of how the existing developers run would be an improvement

The discussion in #842 seems relevant, also (and is what I tried first, but I had the same dm-mapper outcome when using VBox and I don't know enough qemu flags to know how to boot it up)