docs: add paper citation to docs

Author: cadenmyers13

doc/source/index.rst

@@ -43,6 +43,18 @@ Acknowledgements
 
 ``diffpy.cmi`` is built and maintained with `scikit-package <https://scikit-package.github.io/scikit-package/>`_.
 
+========
+Citation
+========
+
+If you use ``diffpy.cmi`` in a scientific publication, we would like you to cite this package as
+
+    Juhás, P.; Farrow, C. L.; Yang, X.; Knox, K. R.; Billinge, S. J. L.
+    Complex Modeling: A Strategy and Software Program for Combining Multiple Information Sources to Solve Ill Posed Structure and Nanostructure Inverse Problems.
+    *Acta Crystallogr A Found Adv* **2015**, *71* (6), 562–568.
+    `https://doi.org/10.1107/S2053273315014473 <https://doi.org/10.1107/S2053273315014473>`_
+
+
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 Table of contents
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