Add short description of AKA domains
Add short description of AKA domains

They are necessary if you want to use the [side-by-side browser](https://github.com/alphagov/side-by-side-browser)
when transitioning a site.

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