Document need for enabling linger-mode

Signed-off-by: Chris Evich <cevich@redhat.com>

README.md

@@ -38,7 +38,19 @@ $ IMAGE="registry.gitlab.com/qontainers/pipglr:latest"
 $ eval $(podman inspect --format=json $IMAGE | jq -r .[].Labels.<label>)
 ```
 
-#### Runner registration (step 1)
+#### Persistent containers (step 1)
+
+By default on many distributions, regular users aren't permitted to leave
+background processes running after they log out.  Since this is likely
+desired for the user running the pipglr container, `systemd` needs to be
+configured to override this policy.  For this, you (`$USER`) will need
+root access on the system.
+
+```bash
+$ sudo loginctl enable-linger $USER
+```
+
+#### Runner registration (step 2)
 
 All runners must be connected to a project or group runner configuration
 on your gitlab instance (or `gitlab.com`).  This is done using a special
@@ -75,7 +87,7 @@ $ podman container runlabel register $IMAGE
 $ podman secret rm REGISTRATION_TOKEN  # if desired
 ```
 
-#### Runner Configuration (step 2)
+#### Runner Configuration (step 3)
 
 During the registration process (above), a boiler-plate (default) `config.toml` file
 will be created/updated for you.  At this point you may edit the configuration
@@ -94,7 +106,7 @@ to reflect the number of registered runners.  If you need to edit this file
 after commiting it as a secret, there's
 [ a `dumpconfig`  *runlabel* for that](README.md#configuration-editing).
 
-#### Volume setup (step 3)
+#### Volume setup (step 4)
 
 Since several users are utilized inside the container volumes must be
 specifically configured to permit access.  This is done using several
@@ -112,7 +124,7 @@ it's running low.  Simply remove them with the command
  `podman volume rm pipglr-storage pipglr-cache`.  The reuse the `setupstorage`
 and `setupcache` *runlabels* as in the above example.
 
-#### Runner Startup (step 4)
+#### Runner Startup (step 5)
 
 With the runner configuration saved as a Podman secret, and the runner volumes
 created, the GitLab runner container may be launched with the following commands: