Add reference paper to README.

README.md

@@ -352,3 +352,8 @@ See the [license page on the Open XDMoD website][license-page] for details.
 [xdmod-qa]: https://github.com/ubccr/xdmod-qa
 [xdmod-repo-manifest]: https://github.com/ubccr/xdmod-repo-manifest
 [software-requirements]: http://open.xdmod.org/software-requirements.html
+
+## Reference
+When referencing XDMoD, please cite the following publication:
+
+> Jeffrey T. Palmer, Steven M. Gallo, Thomas R. Furlani, Matthew D. Jones, Robert L. DeLeon, Joseph P. White, Nikolay Simakov, Abani K. Patra, Jeanette Sperhac, Thomas Yearke, Ryan Rathsam, Martins Innus, Cynthia D. Cornelius, James C. Browne, William L. Barth, Richard T. Evans, "Open XDMoD: A Tool for the Comprehensive Management of High-Performance Computing Resources", *Computing in Science & Engineering*, Vol 17, Issue 4, 2015, pp. 52-62. DOI:10.1109/MCSE.2015.68