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.
Let's add a little info pointing users to the UI:

To get started adding webhooks, go to the desired repo in the Hub, and click "Webhooks" under the "Settings" box.