Include citation in readme

README.md

@@ -21,3 +21,7 @@ Understanding the spatio-temporal patterns of emergence and circulation of new h
 * [Antigenic phenotype for B/Victoria viruses](antigenic/)
 * [Epidemic dynamics across viruses in USA and Australia](timeseries/)
 * [Age structure of airline travel and influenza infections](age-structure/)
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+## Citation
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+[Bedford T, Riley S, Barr IG, Broor S, Chadha M, Cox NJ, Daniels RS, Gunasekaran CP, Hurt AC, Kelso A, Klimov A, Lewis NS, Li X, McCauley JW, Odagiri T, Potdar V, Rambaut A, Shu Y, Skepner E, Smith DJ, Suchard MA, Tashiro M, Wang D, Xu X, Lemey P, Russell CA. 2015. Global circulation patterns of seasonal influenza viruses vary with antigenic drift. Nature 523: 217-220.](http://dx.doi.org/10.1038/nature14460)