Add payload description

LoRaWAN_Payload_Description.md

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+# LoRaWAN Payload Description - Raumsensor Device
+
+## Uplink Payload (Device → Network Server)
+
+**Port:** 2
+**Length:** 7 bytes
+**Format:** Binary
+
+| Byte | Field | Type | Unit | Description |
+|------|-------|------|------|-------------|
+| 0-1 | Room Temperature | int16_t | 0.01°C | SHT4x sensor reading (big-endian) |
+| 2-3 | Floor Temperature | int16_t | 0.01°C | MLX90614 IR sensor reading (big-endian) * |
+| 4-5 | Humidity | int16_t | 0.01% | SHT4x sensor reading (big-endian) |
+| 6 | Relay State | uint8_t | - | Current relay status (0=OFF, 1=ON) |
+
+**Example Uplink:**
+- `08 34 0A 28 15 E0 01` = 21.00°C room, 26.00°C floor, 56.00% humidity, relay ON
+
+## Downlink Payload (Network Server → Device)
+
+**Port:** Any
+**Format:** ASCII commands
+
+### Relay Control Commands
+
+| Command | Length | Format | Description |
+|---------|--------|--------|-------------|
+| `r1 0` | 4 bytes | ASCII | Force relay OFF + disable auto control |
+| `r1 1` | 4 bytes | ASCII | Re-enable automatic temperature control |
+
+### Temperature Threshold Commands
+
+| Command | Length | Format | Description |
+|---------|--------|--------|-------------|
+| `t1[temp]` | 3 bytes | ASCII + uint8 | Set room temperature threshold (°C) |
+| `t2[temp]` | 3 bytes | ASCII + uint8 | Set floor temperature threshold (°C) |
+
+**Examples:**
+- `72 31 20 30` (`r1 0`) = Force relay OFF
+- `72 31 20 31` (`r1 1`) = Enable auto control
+- `74 31 16` (`t1` + 22) = Set room threshold to 22°C
+- `74 32 1A` (`t2` + 26) = Set floor threshold to 26°C
+
+## Data Conversion
+
+- **Temperature:** `value = (int16_t)(temperature * 100)`
+- **Humidity:** `value = (int16_t)(humidity * 100)`
+- **Decoding:** `actual_value = received_value / 100.0`
+
+## Special Cases
+
+**\* Floor Temperature Sensor:** If the MLX90614 sensor is not available or not responding, the floor temperature will be transmitted as `-273.15°C` (hex: `95 5D`).