citation added to index of docs

Author: sbillinge

Diff for: doc/source/index.rst

@@ -49,6 +49,14 @@ Installation
 See the `README <https://github.com/diffpy/diffpy.utils#installation>`_
 file included with the distribution.
 
+========
+Citation
+========
+
+If you use this program for a scientific research that leads to publication, we ask that you acknowledge use of the program by citing the following paper in your publication:
+
+    P. Juhás, C. L. Farrow, X. Yang, K. R. Knox and S. J. L. Billinge, Complex modeling: a strategy and software program for combining multiple information sources to solve ill posed structure and nanostructure inverse problems, Acta Crystallogr. A 71, 562-568 (2015).
+
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 Table of contents
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