contrib/install.sh: Add details to the success message

First warn the user that balena-engine-daemon needs to be started.
Including instructions on how to make the system ready for that:
- service files
- balena-engine group
- how to allow non-root users to run containers

Connects-to: #55
Connects-to: #51
Signed-off-by: Robert Günzler <robertg@balena.io>

contrib/install.sh

@@ -52,4 +52,24 @@ cat <<EOF
  /_.___/\\__,_/_/\\___/_/ /_/\\__,_/_____/_/ /_/\\__, /_/_/ /_/\\___/
                                             /____/
   the container engine for the IoT
+
+To use balenaEngine you need to start balena-engine-daemon as a background process.
+This can be done manually or using the init system scripts provided here:
+
+    https://github.com/balena-os/balena-engine/tree/$tag/contrib/init
+
+This requires adding a \"balena-engine\" group for the daemon to run under:
+
+    sudo groupadd -r balena-engine
+
+If you want to allow non-root users to run containers they can be added to this group
+with something like:
+
+    sudo usermod -aG balena-engine <user>
+
+WARNING: Adding a user to the \"balena-engine\" group will grant the ability to run
+         containers which can be used to obtain root privileges on the
+         docker host.
+         Refer to https://docs.docker.com/engine/security/security/#docker-daemon-attack-surface
+         for more information.
 EOF