Add description of retrieving parameters (#23)

Add description of retrieving parameters from other nodes in rclpy via
services.

---------

Co-authored-by: Michael Ferguson <[email protected]>

rclpy/parameters.md

@@ -67,3 +67,47 @@ class MyNode(Node):
 
 For an example of calling the set_parameters service, see
 [ROS Answers](https://answers.ros.org/question/308541/ros2-rclpy-set-parameter-example/)
+
+## Getting parameters from other nodes
+
+In ros2, this is done through a service call. For example, to get the `robot_description` parameter published by `robot_state_publisher`:
+
+```python
+from rcl_interfaces.srv import GetParameters
+
+import rclpy
+from rclpy.node import Node
+
+class GetParam(Node):
+    def __init__(self):
+        super().__init__('joint_command_publisher')
+        self.client = self.create_client(GetParameters, '/robot_state_publisher/get_parameters')
+        robot_description = self.get_robot_description()
+
+    def get_robot_description(self):
+        request = GetParameters.Request()
+        request.names = ["robot_description"]
+        future = self.client.call_async(request)
+        rclpy.spin_until_future_complete(self, future)
+        if future.result() is not None:
+            self.get_logger().info(future.result().values[0].string_value )
+            return future.result().values[0].string_value  # Assuming the parameter is a string, adjust as necessary
+        else:
+            self.get_logger().error('Failed to call service')
+            return None
+
+def main(args=None):
+    rclpy.init(args=args)
+    node = GetParam()
+
+    try:
+        rclpy.spin(node)
+    except KeyboardInterrupt:
+        pass
+
+    node.destroy_node()
+    rclpy.shutdown()
+
+if __name__ == '__main__':
+    main()
+```