describe config-extend in what's new

docs/source/whatsnew/pr/config-extend.rst

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+Extending Configurable Containers
+---------------------------------
+
+Some configurable traits are containers (list, dict, set)
+Config objects now support calling ``extend``, ``update``, ``insert``, etc.
+on traits in config files, which will ultimately result in calling
+those methods on the original object.
+
+The effect being that you can now add to containers without having to copy/paste
+the initial value::
+
+    c = get_config()
+    c.InlineBackend.rc.update({ 'figure.figsize' : (6, 4) })
+
+