Add MIT license

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -145,3 +145,10 @@ After 3 wrong guesses, the key also needs unplugging before it accepts more.
 2. hmac-secret / prf extension
 3. Pico 2: keep the master secret in OTP and enable secure boot
 4. Optional external button on a GPIO pin
+
+## License
+
+MIT; see [LICENSE](LICENSE). Firmware builds also include the Pico SDK
+(BSD-3-Clause), TinyUSB (MIT) and Mbed TLS (Apache-2.0 or GPL-2.0-or-later),
+each under its own
+license.