Add a MQTT sensor example for a DHT22 sensor (#773)

source/_components/sensor.mqtt.markdown

@@ -61,3 +61,33 @@ sensor:
   value_template: {% raw %}'{{ value_json.batt }}'{% endraw %}
 ```
 
+### {% linkable_title Get temperature and humidity %}
+
+If you are using a DHT sensor and a NodeMCU board (esp8266), you can retrieve temperature and humidity with a MQTT sensor. A code example can be found [here](https://github.com/mertenats/open-home-automation/tree/master/ha_mqtt_sensor_dht22). A regular MQTT message from this example looks like this: 
+
+```
+office/sensor1
+  {
+    "temperature": 23.20,
+    "humidity": 43.70
+  }
+```
+
+Then use this configuration example to extract the data from the payload:
+
+```yaml
+# Example configuration.yml entry
+sensor 1:
+  platform: mqtt
+  state_topic: 'office/sensor1'
+  name: 'Temperature'
+  unit_of_measurement: '°C'
+  value_template: '{{ value_json.temperature }}'
+
+sensor 2:
+  platform: mqtt
+  state_topic: 'office/sensor1'
+  name: 'Humidity'
+  unit_of_measurement: '%'
+  value_template: '{{ value_json.humidity }}'
+```