Caveat about using the same DNS provider as GDS

I wasn't sure whether this is appropriate for the service manual, as it
references a particular provider (I have avoided hyperlinking to avoid the
idea that we are promoting them over any other).

We have an issue when Transactional Services want to use the same DNS provider
as GDS uses to manage the service.gov.uk domain. In order for the backend
systems to know which customer account is authorative for that particular
record, the provider needs to make some alterations and requires a signed
letter of authorisation from both parties.

As this is outside of our control and may affect timescales, I thought it
worth documenting here.

/cc @daibach