Add GCP Cloud Build deployment README (#62)

Author: roger2hk

deployment/live/gcp/cloudbuild/README.md

@@ -0,0 +1,24 @@
+# GCP Cloud Build Triggers and Steps
+
+This directory contains terragrunt files to configure our Cloud Build pipeline(s).
+
+The Cloud Build pipeline is triggered on commits to the `main` branch of the repo, and
+is responsible for:
+
+1. Building the `cmd/gcp` and `cmd/gcp/ci` docker images from the `main` branch,
+1. Deploying the `cmd/gcp/ci` image to Cloud Run,
+1. TODO: Creating a fresh conformance testing environment,
+1. TODO: Running the conformance test against the newly build conformance docker image,
+1. TODO: Turning-down the conformance testing environment.
+
+## Initial setup
+
+The first time this is run for a pair of {GCP Project, GitHub Repo} you will get an error 
+message such as the following:
+
+```
+Error: Error creating Trigger: googleapi: Error 400: Repository mapping does not exist. Please visit $URL to connect a repository to your project
+```
+
+This is a manual one-time step that needs to be followed to integrate GCP Cloud Build 
+and the GitHub repository.