First pass on new domain name/service.gov.uk guidance

We're getting lots of enquiries about the details of getting and running a
service.gov.uk domain. This is a first pass at restructuring our guidance
about those domains to make some details clearer, flesh out some points and
to begin to make it clear that the approvals are part of a wider process.

So far this is very sketchy and intended primarily to move an existing
conversation out of emails/google docs and into 'code'
Update index.md

Minor changes to phrasing
Add more explanation about HSTS and reformat to shorter lines
Add a little more explanation of email origin and testing
Link to getting a name from managing a domain
Add email address for approvals to get the process going
Clarify the information required for dns

We need the following information to arrange DNS delegation
1. Service Name
2. Name Servers
3. Implementation date
Clarify verification of SSL certificates

- List out the possible methods of verification
- Note that @service.gov.uk is not possible
- Ask for warning before email verification
Merge pull request #196 from alphagov/restructure-service-gov-uk

Restructure service.gov.uk guidance