Update README.md

Author: meteoswiss-mdr

README.md

@@ -58,6 +58,13 @@ Suggestions of developments and bug reports should use the [Issues page of the g
 
 The process to contribute by partners external to MeteoSwiss is described in the [user manual](./doc/pyrad_user_manual.pdf).
 
+# Citation
+The core of Pyrad is based on Py-ART. Py-ART was originally developed in the context of the [ARM Research Facility](https://www.arm.gov/). If you use Pyrad for your work, please cite BOTH Py-ART and Pyrad papers in your paper:
+
+J.J. Helmus, S.M. Collis, (2016). The Python ARM Radar Toolkit (Py-ART), a Library for Working with Weather Radar Data in the Python Programming Language. Journal of Open Research Software. 4(1), p.e25. DOI: http://doi.org/10.5334/jors.119
+
+J. Figueras i Ventura, A. Leuenberger, Z. Künsch, J. Grazioli, U. Germann, Pyrad: a Real-Time Weather Radar Data Processing Framework Based on Py-ART, 38th AMS Conference on Radar Meteorology, Chicago, IL, USA, 28 Aug-1 Sept. 2017
+
 # Disclaimer
 The software is still in a development stage. Please let us know if you would like to test it.