add incompat in whats new

docs/source/whatsnew/pr/incompat-remove-js-container.rst

@@ -0,0 +1,4 b''
+Accessing the `container` DOM object when displaying javascript has been
+deprecated in IPython 2.0 in favor of accessing `element`. Starting with
+IPython 3.0 trying to access `container` will raise an error in browser
+javascript console.