Update 'README.md'

Adding more ros notes

README.md

@@ -15,3 +15,190 @@ In a third window
 ```
 rosrun simple_example drive_node.py
 ```
+
+## Nodes and Topics
+
+```
+rostopic bw     display bandwidth used by topic
+rostopic echo   print messages to screen
+rostopic hz     display publishing rate of topic    
+rostopic list   print information about active topics
+rostopic pub    publish data to topic
+rostopic type   print topic type
+```
+
+In a fourth window you can see the active nodes:
+```
+ubuntu@rospi:~/catkin_ws$ rosnode list
+/keyb_commander_1845_1645809765799
+/motor_driver_1870_1645809783593
+/rosout
+```
+
+You can look at the topics used in the deployment:
+
+```
+ubuntu@rospi:~/catkin_ws$ rostopic list -v
+
+Published topics:
+ * /rosout_agg [rosgraph_msgs/Log] 1 publisher
+ * /command [std_msgs/String] 1 publisher
+ * /rosout [rosgraph_msgs/Log] 2 publishers
+
+Subscribed topics:
+ * /rosout [rosgraph_msgs/Log] 1 subscriber
+ * /command [std_msgs/String] 1 subscriber
+```
+
+We can see the publisher and subscriber of a topic and it's message type:
+```
+ubuntu@rospi:~/catkin_ws$ rostopic info /command
+Type: std_msgs/String
+
+Publishers:
+ * /keyb_commander_1845_1645809765799 (http://rospi:35435/)
+
+Subscribers:
+ * /motor_driver_1870_1645809783593 (http://rospi:34943/)
+```
+
+We can see the topic type, which is this case is one that uses `std_msgs/String`
+```
+ubuntu@rospi:~/catkin_ws$ rostopic type /command
+std_msgs/String
+```
+
+If we want to look at the definition of a message we can use `rosmsg`:
+```
+rosmsg show geometry_msgs/Twist
+```
+
+Slam together the `rostopic type` command with `rosmsg show` command to immediately get the message type info
+```
+rostopic type /command | rosmsg show
+```
+
+We can also publish a message to the /command topic like so `rostopic pub [topic] [msg_type] [args]`
+
+```
+rostopic pub /command std_msgs/String "left"
+```
+
+We can also echo out the messages received by the topic:
+
+```
+ubuntu@rospi:~/catkin_ws$ rostopic echo /command
+data: "forward"
+---
+data: "back"
+---
+data: "left"
+---
+data: "right"
+---
+data: "back"
+---
+data: "forward"
+---
+```
+
+If we want to look at descriptive stats of the rate of messages published to a topic we can use the hertz subcommand:
+```
+rostopic hz /command
+```
+
+## Parameters and Services
+
+### Services
+
+From [Understanding ROS Parameters and Services]
+Services are synchronous (blocking) RPC servers essentially
+
+```
+rosservice list         print information about active services
+rosservice call         call the service with the provided args
+rosservice type         print service type
+rosservice find         find services by service type
+rosservice uri          print service ROSRPC uri
+```
+
+```
+ubuntu@rospi:~/catkin_ws$ rosservice list
+/keyb_commander_1845_1645809765799/get_loggers
+/keyb_commander_1845_1645809765799/set_logger_level
+/motor_driver_1870_1645809783593/get_loggers
+/motor_driver_1870_1645809783593/set_logger_level
+/rosout/get_loggers
+/rosout/set_logger_level
+```
+
+In this package we have no services available beyond the basic logging facilities.
+
+We cab get the type of service messages that they expect much like the rostopics.
+```
+rosservice type /keyb_commander_1845_1645809765799/get_loggers
+roscpp/GetLoggers
+```
+
+If we have `std_srvs/Empty` as a message type like in `/turtlesim` we would call that service with no arguments
+using the `rosservice call /clear` command. Other services might look like this:
+
+```
+toor@ubuntu:~/workspace/ros/src$ rosservice type /spawn | rossrv show
+float32 x
+float32 y
+float32 theta
+string name
+---
+string name
+```
+
+```
+toor@ubuntu:~/workspace/ros/src$ rosservice call /spawn 2 2 0.2 ""
+name: "turtle2"
+```
+
+Spawns a second turtle
+
+### Parameters
+
+ROS provides a parameter server that you read and write values to.
+
+```
+rosparam set            set parameter
+rosparam get            get parameter
+rosparam load           load parameters from file
+rosparam dump           dump parameters to file
+rosparam delete         delete parameter
+rosparam list           list parameter names
+```
+
+Let's just look at all the parameters.
+```
+toor@ubuntu:~/workspace/ros/src$ rosparam get /
+rosdistro: 'noetic
+
+  '
+roslaunch:
+  uris:
+    host_ubuntu__38527: http://ubuntu:38527/
+rosversion: '1.15.14
+
+  '
+run_id: a50b0b18-9666-11ec-b706-21c9002ce246
+turtlesim:
+  background_b: 255
+  background_g: 86
+  background_r: 150
+```
+
+## Ros Utility Programs
+
+`rqt_plot` displays a scrolling time plot of the data published on topics. It's a GUI that only works on ros-desktop.
+
+```
+rosrun rqt_plot rqt_plot
+```
+
+
+[Understanding ROS Parameters and Services](https://wiki.ros.org/ROS/Tutorials/UnderstandingServicesParams