Expanded documentation for Webhooks 2.0

Docs people: @fredlf @SvenDowideit 
Hub people: @kencochrane @fxdgear

Please review! Also note that this feature isn't in production yet but is slated for release in a few days.
Using "here" for link text is not good web design practice. Let's recast to make the link clear: 

See the Docker Hub documentation for [more information on webhooks](http://docs.docker.com/docker-hub/repos/#webhooks).