message: Adding some beginner advice to the README.
diff: A compiler for generating TensorFlow numeric and control flow ops from Python
  code.
  +
  +### Eng Guide
  +See tensorflow/contrib/py2tf/impl/api.py for the decorator definition and entry
  +point to the conversion code.
  +
  +See tensorflow/contrib/py2tf/impl/conversion.py for where all of the
  +`Transformer`s are called on the AST.
  +
  +In order to alter the AST one should create a subclass of `transformer.Base`, as
  +seen in converters/. In this subclass if one wants to add code that runs on
  +each node then the `visit_<node_name>` method should be overridden, where
  +`<node_name>` is the name of the type of node you wish to alter. See
  +https://docs.python.org/2/library/ast.html#ast.NodeTransformer and note that we
  +use gast to bridge some Python version differences. Also
  +http://greentreesnakes.readthedocs.io/en/latest/nodes.html has references on
  +which visitation functions are supported. The `visit_<node_name>` function then
  +returns the node that will be included in the final AST. An example of this is
  +the following `Transformer` that will alter all while loops:
  +
  + ```
  + class WhileLoopTransformer(transformer.Base):
  +
  + def __init__(self, context):
  + super(WhileLoopTransformer, self).__init__(context)
  +
  + def visit_While(self, node):
  + return node
  + ```
  +
  +Here, `visit_While` will be called on all while loop nodes with the node passed
  +in as node. Because we just return node without altering it, this is a no-op.
  +
  +One thing to note is that this will not recursively alter nested while loops; in
  +order to do this we need to call `self.generic_visit(node)` which is a
  +pre-defined function that recursively visits all the children of `node`.
  +
  +In order to have the new `Transformer` actually be called on the AST, it needs
  +to be called from `node_to_graph` in
  +tensorflow/contrib/py2tf/impl/conversion.py.
  