[Keyboard] Add Mode Eighty M80v2 PCB support (#15486)

Co-authored-by: Ryan <fauxpark@gmail.com>

keyboards/mode/m80v2/readme.md

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+# Mode Eighty V2 PCB QMK firmware
+
+![MODE80](https://i.imgur.com/26uzM3yl.jpg)\
+[Image Gallery](https://imgur.com/t/mechanicalkeyboards/8Uf6c2m?nc=1)
+
+The Mode80 is a tenkeyless high-end keyboard sold by [Mode Designs](https://shop.modedesigns.com/).
+
+* Keyboard Maintainer: [Gondolindrim](https://github.com/gondolindrim)
+* Hardware Supported: proprietary PCB using STM32F072 controller
+* Hardware Availability: you can get a Mode Eighty as of today (dec. 2021) through the in stock sales or special groupbuy editions at https://shop.modedesigns.com/
+
+## How to flash
+
+### Enter bootloader
+
+The DFU state in the bootloader can be accessed in 3 ways:
+
+* **Bootmagic reset**: Hold down the key at (0,0) in the matrix (usually the top left key or Escape) and plug in the keyboard
+* **Physical reset button**: press the button on the front of the PCB, next to caps lock, for at least five seconds
+* **Keycode in layout**: Press the key mapped to `RESET` if it is available (ESC key at layer 1 in the default layout)
+
+## Compile firmware
+
+After accessing the DFU state in your PCB, build the default layout using:
+
+    make mode/m80v2/m80v2h:default # Hotswap
+    make mode/m80v2/m80v2s:default # Soldered
+
+Then download the resulting binary `*bin` file using `dfu-util` or QMK Toolbox.
+
+## Compile and flash
+
+After accessing the DFU state in your PCB, you can compile-and-flash automatically usind `dfu-util` through
+
+    make mode/m80v2/m80v2h:default:flash # Hotswap
+    make mode/m80v2/m80v2s:default:flash # Soldered
+
+See the [build environment setup](https://docs.qmk.fm/#/getting_started_build_tools) and the [make instructions](https://docs.qmk.fm/#/getting_started_make_guide) for more information. Brand new to QMK? Start with our [Complete Newbs Guide](https://docs.qmk.fm/#/newbs).