Add Continuous Delivery section

Embarassing, MVP version. This probably needs rewriting a few times
but I'm sharing to allow others to collaborate and improve.
Add more context around security issues
Apply style guide

Use "you" rather than "one".
Add links

Links are kind of a big deal.

* Agile link within the service manual
* Lean philosophy on Wikipedia
* configuration management within the service manual
This is relevant to the "Web Operations" role

..which for some reason is identified by "tech-archs"
Fix syntax of markdown reference-style links
add section about deployment pipeline
Merge pull request #60 from alphagov/continuous-delivery

Add Continuous Delivery section