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>
5 jobs for redpine-new-firmware in 1 minute and 20 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Package
passed #340113
librem5
package:deb-debian-buster

00:00:50

passed #340111
librem5
package:deb-debian-buster

00:00:48

 
  Test Package
passed #340115
librem5
lintian-debian-buster-package

00:00:29

failed #340114
librem5
lintian-debian-buster-package

00:00:31

passed #340112
librem5
lintian-debian-buster-package

00:00:28