[Keyboard] Add M64 RGB (#17069)

keyboards/mss_studio/m64_rgb/readme.md

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+# M64 RGB
+
+Support STM32F072CBU6 keyboard.
+
+* Keyboard Maintainer: [HorrorTroll](https://github.com/HorrorTroll)
+* Hardware Supported: STM32F072CBU6
+
+Make example for this keyboard (after setting up your build environment):
+
+    make mss_studio/m64_rgb:default
+
+Flashing example for this keyboard:
+
+    make mss_studio/m64_rgb:default:flash
+
+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 (Esc key) and plug in the keyboard
+* **Physical reset button**: Press and hold the button on the back of the PCB, then plug in the keyboard
+* **Keycode in layout**: Press the key mapped to `RESET` if it is available