README: Refactor explanation of how branches work
README: Refactor explanation of how branches work

Remove the <branch> variable from the instructions of how to build with
vagrant since it is no longer used.

Add description of how branches work when building using vagrant.

Add description of how branches work when building using repo tool.

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

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