Added MQTT.md

client/MQTT.md

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+ # MQTT Configuration
+
+ This project can publish weather and public transport data to an MQTT broker.
+ To enable MQTT, follow these steps:
+
+ ## 1. Install dependencies
+ Ensure you have Rust and Cargo installed. The MQTT support uses the Paho MQTT client crate.
+ Run:
+ ```bash
+ cargo update
+ ```
+
+ ## 2. Set the MQTT_BROKER environment variable
+ Before running the client, define `MQTT_BROKER` to your broker address.
+ - Without URI scheme (defaults to TCP):
+   ```bash
+   export MQTT_BROKER=localhost:1883
+   ```
+ - With URI scheme:
+   ```bash
+   export MQTT_BROKER=tcp://broker.example.com:1883
+   ```
+
+ ## 3. Run the LED board client
+ Pass the LED board IP address as the only argument:
+ ```bash
+ export MQTT_BROKER=localhost:1883
+ cargo run --bin ledboard_client -- 192.168.1.50
+ ```
+
+ ## Topics and Payloads
+ The client publishes two topics:
+
+ ### weather
+ JSON payload with fields:
+ - `dt`: timestamp (Unix seconds)
+ - `temp`: temperature in °C
+ - `weather`: object with `main`, `description`, `icon`
+ - `rain`: rain volume in last 3h (optional)
+ - `pop`: probability of precipitation
+ - `wind`: object with `speed`, `deg`, `gust`
+
+ ### straba
+ JSON payload with fields:
+ - `outbound_station`: name of outbound station
+ - `outbound_diff`: seconds until outbound departure
+ - `inbound_station`: name of inbound station
+ - `inbound_diff`: seconds until inbound departure
+
+ ## Customization
+ You can adjust MQTT topics, QoS, and message formats in `client/bin/src/main.rs` under the `publish_to_mqtt` function.