Mention update-all.sh in the README.md
Mention update-all.sh in the README.md

It's a convenience script that runs all of the scripts that keep
the applications and the VM up to date.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet