moar README

Signed-off-by: Jason Hall <jason@chainguard.dev>

image-workflow/README.md

@@ -2,12 +2,55 @@
 
 This sets up a Cloud Run app to listen for `registry.push` events to a private Chainguard Registry group, and triggers a GitHub Actions workflow with that image ref as an input.
 
-The Terraform does everything:
+To start, create a GitHub workflow at `.github/workflows/workflow.yaml`, with an input named `image`:
 
-- builds the mirroring app into an image using `ko_build`
-- deploys the app to a Cloud Run service
+```
+on:
+  workflow_dispatch:
+    inputs:
+      image:
+        description: 'Image to test'
+        required: true
 
-`TODO: package as a module like enforce-events`
+jobs:
+  test-image:
+    runs-on: ubuntu-latest
+    steps:
+      - run: |
+          # Your tests go here.
+          echo ding ding testing ${{ github.event.inputs.image }}
+```
+
+Then Terraform apply the module (e.g., from the root of this repo):
+
+```
+module "image-workflow" {
+  source = "./image-workflow"  # TODO: move to enforce-events
+
+  # name is used to prefix resources created by this demo application
+  # where possible.
+  name = "chainguard-dev"
+
+  # This is the GCP project ID in which certain resource will live including:
+  #  - The container image for this application,
+  #  - The Cloud Run service hosting this application,
+  project_id = "<project-id>"
+
+  # The Chainguard IAM group from which we expect to receive events.
+  # This is used to authenticate that the Chainguard events are intended
+  # for you, and not another user.
+  # Images pushed to repos under this group will trigger workflows.
+  group = "<group-id>"
+
+  # These describe the GitHub organization, repository and workflow to trigger.
+  github_org         = "my-org"
+  github_repo        = "my-repo"
+  github_workflow_id = "workflow.yaml"
+
+  # Location of the Cloud Run subscriber.
+  # location = "us-central1" (default)
+}
+```
 
 Once things have been provisioned, this module outputs a `secret-command`
 containing the command to run to upload your Github "personal access token" to