added a ref to theano Scipy paper.

doc/index.txt

@@ -44,6 +44,8 @@ You can download the latest `PDF documentation <http://deeplearning.net/software
 
 Check out how Theano can be used for Machine Learning: `Deep Learning Tutorials <http://www.deeplearning.net/tutorial>`_.
 
+A `Scipy conference paper <http://www.iro.umontreal.ca/~lisa/publications2/index.php/publications/show/461>`_ that describe Theano. 
+
 Community
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