Add mention of runner on rain01 to README

README.md

@@ -54,6 +54,9 @@ After following GitLab instructions for registering a new GitLab runner, we
 need to do some extra setup to prepare the runner's account. See the
 `.gitlab-ci.yml` file for the exact commands that are run for each job.
 
+At the time of writing a runner is registered on the `rain01` server with a
+"shell" executor with everything running under the gitlab-runner user account.
+
 ### Add conda to the runner's environment
 
 The GitLab CI configuration expects the runner to know what the "conda"