Update readme

README.md

 # Usage
 
-TBD
+Register a runner with GitLab Runner and configure it like this:
+
+```
+[[runners]]
+  name = "DrupalCI"
+  url = "https://your.gitlab.url/"
+  token = "..."
+  executor = "docker"
+  [runners.docker]
+    tls_verify = false
+    image = "registry.lakedrops.com/docker/gitlab-drupal-ci"
+    privileged = true
+    disable_entrypoint_overwrite = false
+    oom_kill_disable = false
+    disable_cache = false
+    volumes = ["/var/run/docker.sock:/var/run/docker.sock", "/cache"]
+    shm_size = 0
+  [runners.cache]
+    [runners.cache.s3]
+    [runners.cache.gcs]
+```
 
 # Credit