add CYA warning to top of docs

.. MFIX-EXA documentation master file, created Thu Aug  2 12:19:39 2018.

.. warning::

   This documentation is a work in progress. 

   It is reasonably complete and hopefully useful 

   but is likely error prone and in places misleading.

   Please report any errors to the 

   `MFIX-Exa User Forum <https://mfix.netl.doe.gov/forum/c/mfix-exa>`_. 

Welcome to MFIX-EXA's documentation!

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