Added README.md for pendulum_msgs.

pendulum_msgs/README.md

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+## **What Is This?**
+
+The **pendulum_msgs** ROS 2 package is a dependency of **pendulum_control** ROS 2 package. It contains `JointCommand.msg`, `JointState.msg` and `RttestResults.msg`
+
+Please refer to [pendulum_control](https://github.com/ros2/demos/tree/rolling/pendulum_control) for more details.
+
+### **JointCommand.msg**
+
+```msg
+float64 position
+```
+
+### **JointState.msg**
+
+```msg
+float64 position
+float64 velocity
+float64 effort
+```
+
+### **RttestResults.msg**
+
+```msg
+builtin_interfaces/Time stamp
+
+JointCommand command
+JointState state
+
+uint64 cur_latency
+float64 mean_latency
+uint64 min_latency
+uint64 max_latency
+uint64 minor_pagefaults 
+uint64 major_pagefaults
+```
+
+
+## References
+
+- Real-time Jitter Measurements under ROS 2: The Inverted Pendulum Case: https://www.researchgate.net/publication/350353690_Real-time_Jitter_Measurements_under_ROS2_the_Inverted_Pendulum_case