Document email on push service

What does this MR do?

Documents the email on push service.

Are there points in the code the reviewer needs to double check?

Styleguide.

Why was this MR needed?

Lack of documentation on this service.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/80

Screenshots (if relevant)

See changes.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

//cc @stanhu