DOC fix d3viz install instructions for Python 3

doc/library/d3viz/index.txt

@@ -17,12 +17,19 @@ Requirements
 ------------
 
 ``d3viz`` requires the `pydot <https://pypi.python.org/pypi/pydot>`__
-package, which can be installed with ``pip``:
-
-::
+package. If you're using Python 2.x install pydot with pip::
 
     pip install pydot
 
+In Python 3.x install `pydot-ng <https://pypi.python.org/pypi/pydot-ng>`__
+fork instead::
+
+    pip install pydot-ng
+
+Like Theano’s `printing module
+<http://deeplearning.net/software/theano/library/printing.html>`__, ``d3viz``
+requires `graphviz <http://www.graphviz.org/>`__ binary to be available.
+
 Overview
 --------