Doc pydot-ng that work with python 3

doc/install.txt

@@ -63,8 +63,9 @@ The following libraries and software are optional:
     `Git <http://git-scm.com>`_
         To download bleeding-edge versions of Theano.
 
-    `pydot <https://code.google.com/p/pydot/>`_
+    `pydot-ng <https://github.com/pydot/pydot-ng>`_ or `pydot <https://code.google.com/p/pydot/>`_
         To be able to make picture of Theano computation graph.
+	pydot-ng is a pydot compatible replacement that support newer Python.
 
     `NVIDIA CUDA drivers and SDK`_
         Required for GPU code generation/execution on NVIDIA gpus