* apt_list_upgradable: Prepend sudo to suggestion if used in command
* Add apt_upgrade rule
This suggests `apt upgrade` after `apt list --upgradable` if there are
packages to upgrade. It pairs well with the `apt_list_upgradable` rule,
which suggests `apt list --upgradable` after `apt update` if there are
packages to upgrade.
* Add apt_upgrade rule to README
