Remove line about Puppet skeleton

I don't think this adds anything.
Simplify README by referring to deployment README

Instructions on provisioning new machines and deploying Puppet belong in
the `ci-deployment` repository. Link to the README from that repo to
simplify the instructions.
Merge pull request #253 from alphagov/82935274-simplify_readme

[#82935274] Simplify README by referring to deployment README