Edit README.md.

README.md

@@ -1,17 +1,22 @@
 # gdyn-legged-locomotion
-4 legged robot project of 강남Dynamics Lab
+4-Legged Robot Project of 강남Dynamics Lab
 
 ## Depend
 [spawn_robot_tools](https://bitbucket.org/theconstructcore/spawn_robot_tools/src/master/)
 
 ## Refence
 
 ### Robot Models
-We brought several robot models from the other github sites.
+We brought several robot models from the other github sites, however only hyq works in gazebo.
 
 1. [hyq](https://github.com/iit-DLSLab/hyq-description)
 2. [laikago, mini cheetah, quadruped](https://github.com/bulletphysics/bullet3/tree/master/examples/pybullet/gym/pybullet_data)
 3. [opendog](https://github.com/IanTheEngineer/opendog_robot)
 4. [Anymal](https://github.com/leggedrobotics/raisimLib?fbclid=IwAR3bB5PpxuEAAPYyy0g2pWjJZnUjjM8JWR_39wXo_h1kHiNymYhPQieCY0U)
 
-### Papers
+### Papers & Online Materials
+Reference papers.
+
+1. [Construct, ROS Lecture](http://www.theconstructsim.com/ros-projects-create-hopper-robot-gazebo-step-step/)
+2. [MIT Cheetah 3](https://ieeexplore.ieee.org/document/8593885)
+3. [MPC, MIT Cheetah 3](https://www.researchgate.net/publication/330591547_Dynamic_Locomotion_in_the_MIT_Cheetah_3_Through_Convex_Model-Predictive_Control)