Add runit to installation instructions

The bin/background_jobs script in gitlab-ce now optionally uses chpst -P to run sidekiq as a process group leader: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25654

This enables cleaner (and more production-like) termination when sidekiq receives a SIGINT or SIGTERM signal, per: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25669

By installing the runit package, we can ensure that GDK users get the benefits of this, and that they're testing against what production will do (omnibus already runs sidekiq under chpst -P)

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/40396