chore: readme

README.md

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+# CH32-16-CH-EXT
+
+A simple CH32V003-based extension board providing 16 GPIO channels, primarily designed for controlling [Chinese 16-relay modules](https://www.uctronics.com/download/Amazon/U604302_print.pdf) ([archive](https://web.archive.org/web/20220709045418/https://www.uctronics.com/download/Amazon/U604302_print.pdf)) over RS485. Uses 64-byte internal flash page for state persistence.
+
+KiCad hardware design files are in the `hw` directory.
+
+![photo](img/ch32_extension_photo.jpg)
+![render](img/ch32_extension.png)
+
+## Communication Protocol
+
+### Message Format
+
+The RS485 communication uses a simple 4-byte message format:
+
+```
+Byte 0: Board Address (0x01-0xFE, 0xFF for broadcast)
+Byte 1: Command (0x01 for SET_OUTPUTS)
+Byte 2: Data High Byte
+Byte 3: Data Low Byte
+```
+
+### Protocol Constants
+
+```c
+#define BOARD_ADDRESS   0x01        // Default board address
+#define CMD_SET_OUTPUTS 0x01        // Command to set outputs
+#define BROADCAST_ADDR  0xFF        // Broadcast address
+```
+
+### Example Message
+
+To control the outputs, send a 4-byte message:
+
+```rust
+let message = [
+    address,            // Board address (0xFF for broadcast)
+    CMD_SET_OUTPUTS,    // Command
+    (value >> 8) as u8, // Data high byte
+    value as u8,        // Data low byte
+];
+```