Add note about Zendesk tickets relating to email
Add note about Zendesk tickets relating to email

Tickets relating to email notifications can at first be handled by the
1st line support team, so note this in the documentation on manging
email subscribers.

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