[Keyboard] Add hineybush/h87_g2 (#20053)

Co-authored-by: Joel Challis <git@zvecr.com>
Co-authored-by: Drashna Jaelre <drashna@live.com>
Co-authored-by: jack <0x6a73@protonmail.com>

keyboards/hineybush/h87_g2/readme.md

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+# h87_g2
+
+[h87_g2](https://i.imgur.com/t7chDf8h.png)
+
+New generation of the h87 keyboard PCB platform with an STM32 microcontroller.
+
+* Keyboard Maintainer: [Josh Hinnebusch](https://github.com/hineybush)
+* Hardware Supported: H87 G2 PCB w/ STM32F072 MCU
+* Hardware Availability: [hineybush.com](https://hineybush.com)
+
+Make example for this keyboard (after setting up your build environment):
+
+    make hineybush/h87_g2:default
+
+Flashing example for this keyboard:
+
+    make hineybush/h87_g2: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 (usually the top left key or Escape) and plug in the keyboard
+* **Physical reset button**: Briefly press the button on the back of the PCB - some may have pads you must short instead
+* **Keycode in layout**: Press the key mapped to `QK_BOOT` if it is available