start some documentation for the new transformation API

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+Extending Astroid Syntax Tree
+=============================
+
+Sometimes Astroid will miss some potentially important information you may wish
+you add, as content of the standard library `hashlib` module. In some other
+cases, you may want to customize the way inference work, for instance to explain
+Astroid that `collections.namedtuple` is returning a class with some known
+attributes.
+
+The good news is that you can do it using the transformation API. You'll find
+examples in the `brain/` subdirectory. Those come from the `pylint-brain`_ project.
+
+Transformation functions are registered using the `register_transform` method of
+the Astroid manager:
+
+.. automethod:: astroid.manager.AstroidManager.register_transform
+
+You may want to use :class:`astroid.AsStringRegexpPredicate` predicate objects
+to filter on the `as_string` representation of the node.
+
+.. autoclass:: astroid.AsStringRegexpPredicate
+
+Last but not least, the :func:`inference_tip` function is there to register
+custom inference function.
+
+.. autofunction:: astroid.inference_tip
+
+.. _`pylint-brain`: https://bitbucket.org/logilab/pylint-brain
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