Basic RS232 Communication (#780)

* Added ZGateway RS232

docs/use/rs232.md

@@ -0,0 +1,23 @@
+# RS232 gateway
+
+## Sending an RS232 message
+
+Simply publish the message you wish to transmit, minus the prefix and postfix. For example, to send the "Turn On" signal for a Mitsubishi XD221U projector, the code is simply '!' so you would use the command
+
+`mosquitto_pub -t home/OpenMQTTGateway/commands/MQTTtoRS232 -m  '{"value": "!"}'`
+
+It will automatically add the prefix and postfix you set in [config_RS232.h](https://github.com/1technophile/OpenMQTTGateway/blob/master/main/config_RS232.h).
+
+## Receiving an RS232 message
+To receive a message, subscribe to all with `mosquitto_sub -t +/# -v`
+and perform an action that should get a response from the device. For example, If I were to send the "Turn On" signal from earlier, I would receive back
+
+```json
+home/OpenMQTTGateway/RS232toMQTT {"value":"1"}
+```
+
+Because this projector echoes back a received command to acknowledge. Some devices will send a NACK, or Negative Acknowledge, to confirm that they received your message but could not comply. That would look like
+
+```json
+home/OpenMQTTGateway/RStoMQTT {"value":"!:N"}
+```