Mention fork.

README.rst

-============================================================================================================
-`MILA will stop developing Theano <https://groups.google.com/d/msg/theano-users/7Poq8BZutbY/rNCIfvAEAwAJ>`_.
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-
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+Fork of the Theano library which will continue to be maintained for the purpose of supporting PyMC3
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 To install the package, see this page:
    http://deeplearning.net/software/theano/install.html