update README

Author: AtsushiSakai

README.md

@@ -143,12 +143,16 @@ Simultaneous Localization and Mapping(SLAM) examples
 
 ## Iterative Closest Point (ICP) Matching
 
-This is a 2D ICP matching example.
+This is a 2D ICP matching example with singular value decomposition.
 
 It can calculate rotation matrix and translation vector between points to points.
 
 ![3](https://github.com/AtsushiSakai/PythonRobotics/blob/master/SLAM/iterative_closest_point/animation.gif)
 
+Ref:
+
+- [Introduction to Mobile Robotics: Iterative Closest Point Algorithm](https://cs.gmu.edu/~kosecka/cs685/cs685-icp.pdf)
+
 
 # Path Planning