Add some extra details to webhook docs

Update the webhook JSON payloads to real ones,
and show there is a difference between an automated build webhook payload and a normal repo payload

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@docker.com> (github: SvenDowideit)

Signed-off-by: Sven Dowideit <SvenDowideit@docker.com>
What's wrong with something like that?
```
See the Docker Hub documentation for [more information on 
webhooks](http://docs.docker.com/docker-hub/repos/#webhooks)
```