Readme update.

README.md

@@ -33,3 +33,19 @@ package install via an attribute name from `pkgs/default.nix`:
 ```{.bash}
 nix-env -f argopkgs="https://xgitlab.cels.anl.gov/argo/argopkgs/-/archive/master/argopkgs-master.tar.gz" -iA <attribute name>
 ```
+
+## gitlab runners
+
+these are deployed via NixOps:
+
+- defined in file [deployments/ci.nix](deployments/ci.nix)
+- deployment state is in  `deployments/argo.nixops`.
+- there are four shell scripts in [deployments/](deployments/) that can be used to redeploy, reboot, and restart systemd services.
+- access is granted to all keys in [deployment/keys](deployment/keys)
+
+The gitlab-runner service is called "gitlab-runner2" on the machines. That
+systemd service registers runners on ExecStart and unregisters them on
+ExecStop, so excess IPs will have to be merged manually from the gitlab
+[runner
+page](https://xgitlab.cels.anl.gov/groups/argo/-/settings/ci_cd#runners-settings)
+in case runner hosts are rebooted.