Improve README

It is messy

Maybe worth considering #126 (comment 12574):

But perhaps one very small useful simple cross ref link could be to mention the new script emerging on archiso!117 (closed) on https://gitlab.archlinux.org/archlinux/arch-boxes/-/blob/master/README.md#checking-cloud-init-support-in-our-cloud-image when that's available?

mentioned in issue #126 (closed)

mentioned in commit 4b176684

closed with merge request !165 (merged)

closed with commit f72174f4

mentioned in issue #137