add citation

README.md

@@ -4,7 +4,12 @@ Fundamentals of Radiation Effect Group <br>
 Materials Science and Tecnhology Division <br>
 Oak Ridge National Laboratory <br>
  <br>
-Paper submitted to Materials and Design <br>
+Paper published in Materials and Design <br>
+# If you use any of these codes, please cite:<br>
+C.M. Parish and P. D. Edmondson, "Data visualization heuristics for the physical sciences," 
+Materials and Design, <b>V179</b> (2019) #107868<br>
+<br>
+<br>
 `* parishcm@ornl.gov`<br>
 <br>
 tested on Anaconda Python 3.6.8 and 3.7<br>