at requires cron, since atrun is started by cron and won't work without
it, and also requires runtime because at runs jobs under /bin/sh.
bluetooth requires ppp for rfcomm_pppd (a very common use case).
bsdconfig and bsdinstall are written in shell script and therefore
require /bin/sh.
While here, document the policy for adding dependencies in the README.
This will hopefully ensure we end up with consistent dependencies.