Update readme

README.md

 GitLab Runner [![pipeline status](https://git.ethitter.com/ansible/gitlab-runner/badges/master/pipeline.svg)](https://git.ethitter.com/ansible/gitlab-runner/commits/master)
 =============
 
-This role will install the [official GitLab Runner](https://gitlab.com/gitlab-org/gitlab-runner)
-(fork from haroldb) with updates. Needed something simple and working, this did the trick for me. Open for changes though.
+Install GitLab Runner using Docker Machine and Digital Ocean for on-demand instance creation.
+
+This is intended to be installed on the same host as the GitLab instance, spinning up hosts as needed and quickly tearing them down. If your CI activity is frequent, the `gitlab_runner_machine_idle_time` variable should be increased so hosts can be reused.
 
 Requirements
 ------------
@@ -53,6 +54,14 @@ The default Docker image to use. Required when executor is `docker`.
 Default tags assigned to the runner,
 Defaults to an empty list.
 
+###### `gitlab_runner_machine_idle_nodes`
+
+How many idle machines to maintain. Defaults to `0`, meaning all hosts are created on-demand then culled after `gitlab_runner_machine_idle_time` minutes.
+
+###### `gitlab_runner_machine_idle_time`
+
+How long, in minutes, a machine can be idle before it's destroyed. Default is `3`.
+
 ###### `gitlab_runners`
 
 Dict of runners