Initial commit: project docs and ignore rules

README.md

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+# common (shared NFC helpers)
+
+Shared C++ sources for the **dual PN532 ESP-NOW** system described in the [parent README](../README.md): protocol definitions, error handling, memory helpers, and other code consumed by `listener_firmware/` and `emulator_firmware/`.
+
+## Usage
+
+Firmware sketches include headers via paths such as:
+
+```cpp
+#include "../../common/nfc_protocol.h"
+```
+
+Keep this folder versioned **together** with:
+
+- `listener_firmware/`
+- `emulator_firmware/`
+- Parent `platformio.ini` (workspace root)
+
+Splitting into a separate remote is easiest if these three become submodules of a single meta-repo, or you refactor include paths.
+
+## Tests
+
+Unit tests that exercise parts of this tree live in [`tests/`](../tests/).