Added Nonumura reference to README

README.md

 # LoopyWENO
 
-Standalone Fortran WENO code with Loopy transformations
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+Standalone Fortran WENO code with Loopy transformations.  The WENO scheme is
+taken from [1].
+
+## References
+
+[1] Nonomura, T., Terakado, D., Abe, Y., & Fujii, K. (2015).
+*A new technique for freestream preservation of finite-difference WENO on curvilinear grid.*
+Computers and Fluids, 107, 242–255.