Add citation information

README.md

@@ -36,6 +36,22 @@ applications are compiled and tests are skipped.
 Options are specified as -Doption=value, e.g. 
 $ cmake -DUSE_GALOIS=ON ..
 
+## Citation
+
+If you use this code, please cite:
+- Marcin Łoś, Maciej Woźniak, Maciej Paszyński, Andrew 
+Lenharth, Keshav Pingali, IGA-ADS : Isogeometric Analysis FEM using ADS 
+solver, Computer & Physics Communications, 217 (2017) 99-116
+- Marcin Łoś,  Maciej Paszyński, Adriank Kłusek, Witold 
+Dzwinel, Application of fast isogeometric L2 projection solver for tumor 
+growth simulations, Computer Methods in Applied Mechanics and 
+Engineering, 316 (2017) 1257-1269
+- Marcin Łoś,  Adriank Kłusek, M. Amber Hassaan, Keshav 
+Pingali, Witold Dzwinel, Maciej Paszyński,  Parallel fast 
+isogeometric L2 projection solver  with GALOIS system for 3D tumor 
+growth simulations, Computer Methods in Applied
+Mechanics and Engineering, 343, (2019) 1-22
+
 
 ## Contents