Added What's New doc for widgets.

docs/source/whatsnew/pr/widgets.rst

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+Notebook Widgets
+----------------
+
+Available in the new `IPython.html.widgets` namespace, widgets provide an easy
+way for IPython notebook users to display GUI controls in the IPython notebook.
+IPython comes with bundle of built-in widgets and also the ability for users
+to define their own widgets.  A widget is displayed in the front-end using
+using a view.  For example, a FloatRangeWidget can be displayed using a
+FloatSliderView (which is the default if no view is specified when displaying
+the widget).  IPython also comes with a bundle of views and the ability for the
+user to define custom views.  One widget can be displayed multiple times, in on
+or more cells, using one or more views.  All views will automatically remain in
+sync with the widget which is accessible in the back-end.
+
+The widget layer provides an MVC-like architecture on top of the comm layer. 
+It's useful for widgets that can be expressed via a list of properties. 
+Widgets work by synchronizing IPython traitlet models in the back-end with 
+backbone models in the front-end. The widget layer automatically handles
+
+* delta compression (only sending the state information that has changed)
+* wiring the message callbacks to the correct cells automatically
+* inter-view synchronization (handled by backbone)
+* message throttling (to avoid flooding the kernel)
+* parent/child relationships between views (which one can override to specify custom parent/child relationships)
+* ability to manipulate the widget view's DOM from python using CSS, $().addClass, and $().removeClass methods