apply suggestions from review

Co-authored-by: Bastian Lampe <bastian.lampe@rwth-aachen.de>

Author: lreiher

README.md

@@ -61,7 +61,7 @@ The *mqtt_client* is best configured with a ROS parameter *yaml* file. The confi
 - ROS messages received locally on ROS topic `/ping/ros` are sent to the broker on MQTT topic `pingpong/ros`;
 - MQTT messages received from the broker on MQTT topic `pingpong/ros` are published locally on ROS topic `/pong/ros`;
 - primitive ROS messages received locally on ROS topic `/ping/primitive` are sent as primitive (string) messages to the broker on MQTT topic `pingpong/primitive`;
-- MQTT messages received from the broker on MQTT topic `pingpong/primitive` are published locally as primitive ROS messages on ROS topic `/primitive`.
+- MQTT messages received from the broker on MQTT topic `pingpong/primitive` are published locally as primitive ROS messages on ROS topic `/pong/primitive`.
 
 ```yaml
 broker:
@@ -228,15 +228,15 @@ bridge:
 
 ## Primitive Messages
 
-As seen in the [Quick Start](#quick-start), the *mqtt_client* can not only exchange arbitrary ROS messages with other *mqtt_clients*, but it can also exchange primitive message data with other non-*mqtt_client* MQTT clients. The `primitive` parameter can be set for both ROS-to-MQTT (`bridge/ros2mqtt`) and MQTT-to-ROS (`bridge/mqtt2ros`) transmissions.
+As seen in the [Quick Start](#quick-start), the *mqtt_client* can not only exchange arbitrary ROS messages with other *mqtt_clients*, but it can also exchange primitive message data with other non-*mqtt_client* MQTT clients. This allows ROS-based devices to exchange primitive messages with devices not based on ROS. The `primitive` parameter can be set for both ROS-to-MQTT (`bridge/ros2mqtt`) and for MQTT-to-ROS (`bridge/mqtt2ros`) transmissions.
 
-If a ROS-to-MQTT transmission is configured as `primitive`, the ROS message is simply serialized to a string representation, without providing any information of the underlying ROS message type via MQTT. If the ROS message type is one of the supported primitive ROS message types, the encapsualating ROS message components are also removed, s.t. only the raw data is published as a string. The supported primitive ROS message types are [`std_msgs/String`](http://docs.ros.org/en/api/std_msgs/html/msg/String.html), [`std_msgs/Bool`](http://docs.ros.org/en/api/std_msgs/html/msg/Bool.html), [`std_msgs/Char`](http://docs.ros.org/en/api/std_msgs/html/msg/Char.html), [`std_msgs/UInt8`](http://docs.ros.org/en/api/std_msgs/html/msg/UInt8.html), [`std_msgs/UInt16`](http://docs.ros.org/en/api/std_msgs/html/msg/UInt16.html), [`std_msgs/UInt32`](http://docs.ros.org/en/api/std_msgs/html/msg/UInt32.html), [`std_msgs/UInt64`](http://docs.ros.org/en/api/std_msgs/html/msg/UInt16.html), [`std_msgs/Int8`](http://docs.ros.org/en/api/std_msgs/html/msg/Int8.html), [`std_msgs/Int16`](http://docs.ros.org/en/api/std_msgs/html/msg/Int16.html), [`std_msgs/Int32`](http://docs.ros.org/en/api/std_msgs/html/msg/Int32.html), [`std_msgs/Int64`](http://docs.ros.org/en/api/std_msgs/html/msg/Int64.html), [`std_msgs/Float32`](http://docs.ros.org/en/api/std_msgs/html/msg/Float32.html), [`std_msgs/Float32`](http://docs.ros.org/en/api/std_msgs/html/msg/Float64.html).
+If a ROS-to-MQTT transmission is configured as `primitive`, the ROS message is simply serialized to a string representation, without providing any information on the underlying ROS message type via MQTT. If the ROS message type is one of the supported primitive ROS message types, the encapsulating ROS message components are also removed, s.t. only the raw data is published as a string. The supported primitive ROS message types are [`std_msgs/String`](http://docs.ros.org/en/api/std_msgs/html/msg/String.html), [`std_msgs/Bool`](http://docs.ros.org/en/api/std_msgs/html/msg/Bool.html), [`std_msgs/Char`](http://docs.ros.org/en/api/std_msgs/html/msg/Char.html), [`std_msgs/UInt8`](http://docs.ros.org/en/api/std_msgs/html/msg/UInt8.html), [`std_msgs/UInt16`](http://docs.ros.org/en/api/std_msgs/html/msg/UInt16.html), [`std_msgs/UInt32`](http://docs.ros.org/en/api/std_msgs/html/msg/UInt32.html), [`std_msgs/UInt64`](http://docs.ros.org/en/api/std_msgs/html/msg/UInt16.html), [`std_msgs/Int8`](http://docs.ros.org/en/api/std_msgs/html/msg/Int8.html), [`std_msgs/Int16`](http://docs.ros.org/en/api/std_msgs/html/msg/Int16.html), [`std_msgs/Int32`](http://docs.ros.org/en/api/std_msgs/html/msg/Int32.html), [`std_msgs/Int64`](http://docs.ros.org/en/api/std_msgs/html/msg/Int64.html), [`std_msgs/Float32`](http://docs.ros.org/en/api/std_msgs/html/msg/Float32.html), [`std_msgs/Float32`](http://docs.ros.org/en/api/std_msgs/html/msg/Float64.html).
 
 If an MQTT-to-ROS transmission is configured as `primitive`, the MQTT message is interpreted and published as a primitive data type, if possible. The message is probed in the following order: `bool` ([`std_msgs/Bool`](http://docs.ros.org/en/api/std_msgs/html/msg/Bool.html)), `int` ([`std_msgs/Int32`](http://docs.ros.org/en/api/std_msgs/html/msg/Int32.html)), `float` ([`std_msgs/Float32`](http://docs.ros.org/en/api/std_msgs/html/msg/Float32.html)), `string` ([`std_msgs/String`](http://docs.ros.org/en/api/std_msgs/html/msg/String.html)).
 
 ## Latency Computation
 
-The *mqtt_client* provides built-in functionality to measure the latency of transferring a ROS message via an MQTT broker back to ROS. To this end, the sending client injects the current timestamp into the MQTT message. The receiving client can then compute the latency between message reception time and the injected timestamp. **Naturally, this is only accurate to the level of synchronization between clocks on sending and receiving machine.**
+The *mqtt_client* provides built-in functionality to measure the latency of transferring a ROS message via an MQTT broker back to ROS. Note that this functionality is only available for non-primitive messages (see [Primitive Messages](#primitive-messages)). To this end, the sending client injects the current timestamp into the MQTT message. The receiving client can then compute the latency between message reception time and the injected timestamp. **Naturally, this is only accurate to the level of synchronization between clocks on sending and receiving machine.**
 
 In order to inject the current timestamp into outgoing MQTT messages, the parameter `inject_timestamp` has to be set for the corresponding `bridge/ros2mqtt` entry. The receiving *mqtt_client* will then automatically publish the measured latency in seconds as a ROS `std_msgs/Float64` message on topic `/<mqtt_client_name>/latencies/<mqtt2ros/ros_topic>`.