cmd/pushback: update deployment instructions; delete old vm

Change-Id: Ibfdbab5e4c79d3052a860c6e2ce381c140fe4bad
Reviewed-on: https://go-review.googlesource.com/19117
Reviewed-by: Chris Broadfoot <cbro@golang.org>

cmd/pushback/README

@@ -1,5 +1,9 @@
 This service runs as a Google Managed VM.
 Use the gcloud tool to deploy:
 
-$ gcloud --project symbolic-datum-552 preview app deploy --set-default app.yaml
+$ gcloud --project symbolic-datum-552 preview app deploy --no-promote app.yaml
+
+Then use the Cloud Console to set the new version as the default,
+and delete the old instance (lest it keep running):
+https://cloud.google.com/console/appengine/versions?project=symbolic-datum-552&moduleId=pushback