I think we can stop describing the event after a bit:

>  For instance, the [email](https://github.com/github/github-services/blob/master/lib/services/email.rb) service is a built-in GitHub service that will send event [payloads](#payloads)  to, at most, two email addresses.

I don't feel that expanding on the optional headers and other config settings helps to get to the point of what we're talking about: an example of a service.

Also, comma after default: ` event by default, and supports the`