revised README structure

arduino/README.md

@@ -0,0 +1,36 @@
+# Arduino
+
+Sensor interface running on Arduino Nano, communicating with Pi via UART.
+
+## Sketches
+
+| Folder | Purpose |
+|--------|---------|
+| `main/` | Primary telemetry sketch |
+
+## Current Capabilities
+
+- Battery voltage monitoring (voltage divider on A0)
+- Serial output at 9600 baud, 1Hz update rate
+
+## Planned
+
+- RPM sensing (pulse counting from ignition coil)
+- Engine temperature (thermocouple/NTC)
+- Gear position indicator
+- Turn signal / high beam status
+
+## Hardware
+
+- **MCU**: Arduino Nano (ATmega328P)
+- **Connection**: UART to Pi GPIO (TX→RX, RX→TX, common GND)
+- **Voltage sensing**: Resistor divider scaled for 0-20V input range
+
+## Protocol
+
+Simple text-based for now:
+```
+V_bat: 12.45V
+```
+
+Future: structured binary or JSON for multiple sensors.