README.Debian: make instructions more clear

Add a pre-requisites section
Also needs to be run as superuser

Signed-off-by: Angus Ainslie <angus@akkea.ca>
2 jobs for redpine-new-firmware in 1 minute and 16 seconds (queued for 1 second)
Status Job ID Name Coverage
  Package
passed #340109
librem5
package:deb-debian-buster

00:00:47

 
  Test Package
passed #340110
librem5
lintian-debian-buster-package

00:00:28