diff --git a/deployment/modules/gcp/cloudbuild/main.tf b/deployment/modules/gcp/cloudbuild/main.tf
index a3ec2244..fc313cb0 100644
--- a/deployment/modules/gcp/cloudbuild/main.tf
+++ b/deployment/modules/gcp/cloudbuild/main.tf
@@ -77,6 +77,9 @@ resource "google_cloudbuild_trigger" "docker" {
       ]
       wait_for = ["docker_build_example"]
     }
+    // This needs the community-builders terragrunt builder image to be built in this project.
+    // See https://github.com/GoogleCloudPlatform/cloud-builders-community/blob/master/terragrunt/README.markdown
+    // for instructions.
     step {
       id   = "terraform_plan_all"
       name = "cloud-builders-community/terragrunt"