Ref for state lattice planner is added

Author: AtsushiSakai

README.md

@@ -362,6 +362,12 @@ This script is a path planning code with state lattice planning.
 
 This code uses the model predictive trajectory generator to solve boundary problem.
 
+Ref: 
+
+- [Optimal rough terrain trajectory generation for wheeled mobile robots](http://journals.sagepub.com/doi/pdf/10.1177/0278364906075328)
+
+- [State Space Sampling of Feasible Motions for High-Performance Mobile Robot Navigation in Complex Environments](http://www.frc.ri.cmu.edu/~alonzo/pubs/papers/JFR_08_SS_Sampling.pdf)
+
 
 ### Uniform polar sampling