Add a chapter to the book on the layout interface (contract)

[pottedmeat]

People that want to write widgets that can talk to parent widgets. The documentation helps a good deal (http://redesign.dojotoolkit.org/?q=jsdoc/dijit/HEAD/object/dijit.layout._LayoutWidget) but having a front-end in the book explaining this lifecycle would be a huge help, and probably explain the reasons many widgets are in the layout directory.