add build instructions and license to the README

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 # nanji
 
 An experimental OS for the PineTime.
+
+nanji aims to be dependency-free project, handling everything from bootstraping the hardware to the UX.
+Thus, it will not rely on any third party bootloader or external RTOS.
+
+This is a hobby project to play around OS development.
+As such, it is by no way intended to be daily-driven by anyone.
+
+Moreover, I don't even recommend to flash it on any other hardware than the PineTime dev kit wired to a STLink-V2 interface.
+
+nanji is written in C23.
+However, at the time I write this, GCC does not fully support the C23 standard (and as such, can't rely on all new features).
+
+## Building and flashing
+
+Build-time dependencies:
+- GCC cross-compiler (>= 15) and binutils for the `arm-none-eabi` target
+- Make
+- OpenOCD to flash to the PineTime.
+
+To build nanji, you can just run `make` at the project root.
+The resulting binary will be found at `build/nanji`.
+
+Connect your PineTime to your machine through a STLink-V2 interface.
+If you don't know how to do so, please read the [PineTime Devkit Wiring](https://wiki.pine64.org/wiki/PineTime_Devkit_Wiring) guide on the Pine64 wiki.
+
+Once your pinetime is plugged to your PC through STLink-V2, start OpenOCD with:
+
+```sh
+openocd -f /usr/share/openocd/scripts/interface/stlink.cfg -f /usr/share/openocd/scripts/target/nrf52.cfg
+```
+
+You can then connect to the OpenOCD console through telnet or tcl on another shell, and run:
+```
+halt
+nrf5 mass_erase
+program <PATH_TO_THE_NANJI_BINARY>
+```
+
+## License
+
+This project is licensed under the GPLv3.
+See the `LICENSE` file.