docs(gateway): document MQTT_PASSWORD environment variable in README

gateway/README.md

@@ -75,11 +75,16 @@ pip install -r requirements.txt
 
 ## Usage
 
+Set the MQTT password as an environment variable before running:
+
 ```bash
+export MQTT_PASSWORD="your_password"
 source venv/bin/activate
 python gateway.py
 ```
 
+The password is never stored in config files or source code.
+
 ## Notes on the CO2 sensor
 
 The Thingy:52 embeds a CCS811 sensor which measures eCO2 — an estimated