Added HTU21 Sensor (#588)

* Added HTU21 Sensor Support

Co-authored-by: Jonathan Wells <jonathan-wells@oist.jp>

docs/use/sensors.md

@@ -4,7 +4,7 @@ Subscribe to all the messages with mosquitto or open your MQTT client software:
 `    sudo mosquitto_sub -t +/# -v`
 
 ### ADC
-The value is between 0 and 1024 and is tranmitted via Mqtt when it changes. 
+The value is between 0 and 1024 and is tranmitted via Mqtt when it changes.
 `home/OpenMQTTGateway/DHTtoMQTT {"value":543}`
 
 ### DHT
@@ -12,4 +12,11 @@ You will receive every TimeBetweenReadingDHT (set into config_DHT.h) the DHT mea
 
 `home/OpenMQTTGateway/DHTtoMQTT {"temp":21,"hum":51}`
 
-If you want to don't resend value when it is the same you can set dht_always = false in config_DHT.h
+If you want to don't resend value when it is the same you can set dht_always = false in config_DHT.h
+
+### HTU21
+You will receive the HTU21 sensor readings every TimeBetweenReadinghtu21 (set into config_HTU21.h) (30s by default).
+
+`home/OpenMQTTGateway/CLIMAtoMQTT/htu {"tempc":25.34064,"tempf":77.61314,"hum":56.53052}`
+
+If you don't want to resend values that haven't changed you can set htu21_always = false in config_HTU21.h