DOC: Add information on new views for custom Axes.

Author: QuLogic

doc/api/api_changes/2015-08-29-custom-axes-view.rst

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+New API for custom Axes view changes
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+
+Interactive pan and zoom were previously implemented using a Cartesian-specific
+algorithm that was not necessarily applicable to custom Axes. Three new private
+methods, :meth:`~matplotlib.axes._base._AxesBase._get_view`,
+:meth:`~matplotlib.axes._base._AxesBase._set_view`, and
+:meth:`~matplotlib.axes._base._AxesBase._set_view_from_bbox`, allow for custom
+``Axes`` classes to override the pan and zoom algorithms. Implementors of
+custom ``Axes`` who override these methods may provide suitable behaviour for
+both pan and zoom as well as the view navigation buttons on the interactive
+toolbars.