README: Update build with Vagrant
README: Update build with Vagrant

Update description of how to build using Vagrant since this has changed
and is now split into different steps.

Signed-off-by: Viktor Sjölind <de9bdef9475b3a1b942c4473840195297b5e2350@pelagicore.com>

sashko/pelux-manifests,rpannek/pelux-manifests,DunderRoffe/pelux-manifests,Pelagicore/pelux-manifests