Added reference to crack-branching example in README file

README.md

@@ -122,7 +122,7 @@ example can be run with:
 ./CabanaPD/build/install/bin/KalthoffWinkler
 ```
 
-The third example is cracking branching in soda-lime glass. The example can be
+The third example is crack branching in soda-lime glass [3]. The example can be
 run with:
 
 ```
@@ -143,6 +143,8 @@ loading, in Impact Loading and Dynamic Behavior of Materials, C.Y. Chiem, H.-D.
 Kunze, and L.W. Meyer, eds., Vol 1, DGM Informationsgesellschaft Verlag (1988)
 185-195.
 
+[3] F. Bobaru and G. Zhang, Why do cracks branch? A peridynamic investigation of dynamic brittle fracture, International Journal of Fracture 196 (2015): 59–98.
+
 ## Contributing
 
 We encourage you to contribute to CabanaPD! Please check the