Add Mainboard README

Mainboard/README.md

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+# Mainboard
+
+## License
+
+Framework Laptop 12 © 2025 by Framework Computer Inc is licensed under CC BY 4.0.
+To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/
+
+## Pinouts
+
+### Front Panel 9 Pin
+
+Standard Pin layout
+
+```
+┌─┬─┐
+│1│2│
+├─┼─┤
+│3│4│
+├─┼─┤
+│5│6│
+├─┼─┤
+│7│8│
+├─┼─┤
+│9│ │
+└─┴─┘
+```
+
+| Pin | Signal               |
+|-----|----------------------|
+| 1   | +5VALW               |
+| 2   | +5VALW               |
+| 3   | Debug LED (LED4) GND |
+| 4   | Power Button LED GND |
+| 5   | `EC_RST#`            |
+| 6   | Power Button         |
+| 7   | GND                  |
+| 8   | GND                  |
+| 9   | +5VS_Header          |
+
+The power button LED is controlled by the same signal as LED3 on the mainboard. See table below.
+
+`EC_RST#` resets the embedded controller when pulling this signal low.
+After the EC has reset, it does not turn the CPU back on. To do so, use the Power Button (pin).
+
+### ARGB
+
+| Pin | Signal   |
+|-----|----------|
+| 1   | +5VS     |
+| 2   | Data Out |
+| 3   | NC       |
+| 4   | GND      |
+
+Data has 2.2k pullup to +5VS on the mainboard.
+
+## LEDs
+
+All LEDs are controlled by the Embedded Controller firmware.
+
+| LED | Signal | Color | Description                                |
+|-----|--------|-------|--------------------------------------------|
+| 1   | GPIO   | Amber | AC Power (VSB)                             |
+| 2   | GPIO   | Amber | 12V AC Power (G3=Off, S0=On)               |
+| 3   | PWM    | Amber | Power Button (S5=Off, S3=Breathing, S0=On) |
+| 4   | GPIO   | Red   | Debug blinking                             |
+
+Debug blink codes
+
+| Behavior    | Description                        |
+|-------------|------------------------------------|
+| Long blink  | Start of diagnostic                |
+| 1 - On/Off  | PSU OK                             |
+| 2 - On/Off  | 12V OK                             |
+| 3 - On/Off  | CPU deassert sleep S5              |
+| 4 - On/Off  | CPU deassert sleep S4              |
+| 5 - On/Off  | Power boot core VR                 |
+| 6 - On/Off  | CPU Fan Detected                   |
+| 7 - On/Off  | Thermal Sensor Detected            |
+| 8 - On/Off  | CPU reached S0 state               |
+| 9 - On/Off  | DDR initialized OK                 |
+| 10 - On/Off | Power glitch detected during boot  |
+
+## Buttons
+
+| Label | Function       |
+|-------|----------------|
+| SW1   | Clear RTC/CMOS |
+| SW3   | Power Button   |