Add a section about naming service apps
Add a section about naming service apps

These are apps which don't have an API or a web interface and
predominately response to events on the publishing queue.

We're going to formalise the idea of naming apps with a -service
suffix because we're developing a new app similar to
email-alert-service.

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