Add readme and circuit netlist

readme.md

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+# Kulifuli — The Arduino Half
+
+Kulifuli is an Arduino Micro based PC fan controller with the option to send a fake tachometer PWM
+signal back to the motherboard. This is useful for for example server motherboards that refuse to
+operate unless such a signal is received. (Such as the one in my HP Proliant that always spins its
+fans at 100% after the latest firmware update. Thank you HP, very cool of you to do that.)
+
+This repository contains the Arduino side of the software, in addition to information about the
+hardware. For the software to run on the host see
+[this repository](https://kernelmaft.com/forgejo/zedfrigg/kulifuli-host).
+
+## Building
+
+Requirements: GNU Coreutils, GNU Make,
+[Arduino CLI](https://docs.arduino.cc/arduino-cli/),
+[GCC for AVR](https://www.microchip.com/en-us/tools-resources/develop/microchip-studio/gcc-compilers)
+
+This project uses the standard Arduino core library obtained via Arduino CLI, but we do not use it
+as a build tool.
+
+To obtain said library:
+
+```sh
+arduino-cli core update-index
+arduino-cli core install arduino:avr
+```
+
+To build the project:
+
+```sh
+make build
+```
+
+The firmware image for the Arduino Micro should now be present as `build/kulifuli.hex`.
+
+## Flashing
+
+Additional requirement: [AVRDUDE](https://github.com/avrdudes/avrdude)
+
+Connect the Arduino Micro to your computer over USB. If USB serial support is set up correctly the
+device should now have a path it can be referenced by, e.g. `/dev/ttyACM0` on Linux. Create a file
+named `local.make` in the root of the project containing `arduino_serial_port =` followed by this
+path.
+
+Now flash the firmware image to the device:
+
+```sh
+make upload
+```
+
+## Hardware
+
+The hardware design is extremely simple, it contains no active components other than the Arduino
+Micro. Two case fans can be connected to the male fan headers, and the female fan connectors should
+be connected to their original headers on the motherboard. The fan controller sits in between the
+fans and the motherboard, passing through the power and ground but intercepting the control
+signals.
+
+![Circuit diagram of the hardware, see below for a structural representation](docs/circuit.svg)
+
+You can obtain a netlist file of this circuit [here](docs/circuit-netlist.txt).