Add Gentoo Hotswap PCB to QMK (#17528)

keyboards/cannonkeys/gentoo_hs/readme.md

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+# Gentoo65 Hotswap
+
+Gentoo65 Hotswap Keyboard
+
+* Keyboard Maintainer: [Andrew Kannan](https://github.com/awkannan)
+* Hardware Supported: STM32F072CBT6
+
+Make example for this keyboard (after setting up your build environment):
+
+    make cannonkeys/gentoo_hs:default
+
+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).
+
+## Bootloader
+
+Enter the bootloader 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**: Toggle the switch on the back of the pcb to "0" and briefly press the button on the back of the PCB
+* **Keycode in layout**: Press the key mapped to `RESET` if it is available