Add .md to README and clarify what example does

Renamed README to README.md to improve how it renders in an IDE and when
browsing the online repository. Improved the example description to
clarify why the temperature updates might appear unexpectedly sporadic
to the user; and document how to read the current led status.

Author: Martin Crossley

pico_w/wifi/mqtt/README renamed to pico_w/wifi/mqtt/README.md

@@ -34,8 +34,8 @@ When building the code set the host name of the MQTT server, e.g.
 export MQTT_SERVER=myhost
 cmake ..
 ```
-
-The example should publish its core temperature to the /temperature topic. You can subscribe to this topic from another machine.
+The example checks its core temperature every ten seconds and if it has changed, publishes it to the
+/temperature topic. You can subscribe to this topic from another machine.
 
 ```
 mosquitto_sub -h $MQTT_SERVER -t '/temperature'
@@ -47,6 +47,11 @@ You can turn the led on and off by publishing messages.
 mosquitto_pub -h $MQTT_SERVER -t '/led' -m on
 mosquitto_pub -h $MQTT_SERVER -t '/led' -m off
 ```
+You can check the current the state of the led by subscribing to the /led/state topic on another machine.
+```
+mosquitto_sub -h $MQTT_SERVER -t '/led_state'
+```
+
 
 # Security