Add a page on building custom builders

[mattmoor]

Based on: https://cloud.google.com/container-builder/docs/create-custom-build-steps

[knative-housekeeping-robot]

Issues go stale after 90d of inactivity. Mark the issue as fresh with /remove-lifecycle stale. Stale issues rot after an additional 30d of inactivity and eventually close. If this issue is safe to close now please do so with /close. Send feedback to sig-testing, kubernetes/test-infra and/or fejta. /lifecycle stale