Add instructions for authenticating to a registry for the controller

[priyawadhwa]

This way the controller can push signatures to a registry! I got this to work by creating a dockerregistry secret with credentials, and applying it to the tekton-chains-controller service account as an imagePullSecret

$ kubectl create secret docker-registry registry-credentials \
  --docker-server=gcr.io \
  --docker-username=_json_key \
  --docker-email=someemail@something.com \
  --docker-password="$(cat credentials.json)"


$ kubectl patch serviceaccount tekton-chains-controller \
  -p "{\"imagePullSecrets\": [{\"name\": \"registry-credentials\"}]}" -n tekton-chains