Add diagram about A/B testing
Add diagram about A/B testing

I found this diagram in the architecture Google Drive:

https://docs.google.com/drawings/d/1rx4brKZBzj-9q3evkiUw2MbqwqTYWkc0Lku6
u3cLXqU/edit

I'm not sure who created it ("Deleted user", perhaps @carvil?). It
looks accurate so maybe it'll help someone.

alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs