add stm32 dev template

templates/dev/stm32-blink/README.md

@@ -0,0 +1,28 @@
+# STM32G4 blink template
+
+Blinks the LD2 LED (PA5) on a [Nucleo-G474RE](https://www.st.com/en/evaluation-tools/nucleo-g474re.html) using [libopencm3](https://github.com/libopencm3/libopencm3).
+
+Set `BLINK_PORT` / `BLINK_PIN` for your board in [`src/main.c`](./src/main.c).
+
+## Toolchain
+
+The Nix dev shell provides:
+
+- `arm-none-eabi-gcc` (via `gcc-arm-embedded`)
+- `openocd` for flashing and debugging
+- `stlink` utilities (`st-flash`, `st-info`)
+- `libopencm3` built for `stm32/g4` (exposed as `$LIBOPENCM3_DIR`)
+
+## Build
+
+```bash
+make
+```
+
+## Flash
+
+```bash
+make flash
+```
+
+This uses OpenOCD with the built-in ST-Link interface config.