[Keyboard] Add GOAT51 (#18220)

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

keyboards/takashicompany/goat51/readme.md

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+# 🐐 GOAT51
+
+![takashicompany/GOAT51](https://i.imgur.com/7xD9a4zh.jpg)
+
+GOAT51は51キーのキーボードです。
+特徴的な放射状のキーレイアウトは指を自然に置くことができ、効率的な入力が可能です。
+MX互換のキースイッチとChoc v1キースイッチの取り付けに対応しております。
+またPCBにキースイッチソケットを取り付けることが可能で、容易にキースイッチの交換ができます。
+キーボードの中央にはお気に入りのキーキャップを取り付ける専用スペースがあり、アルチザンキーキャップやお気に入りのキーキャップを飾ることができます。
+実はそこにはロータリーエンコーダを置くこともできちゃいます。
+
+---
+GOAT51 is a 51-key keyboard.
+The distinctive radial key layout allows the fingers to be placed naturally for efficient typing.
+It supports the installation of MX compatible key switches and Choc v1 key switches.
+A key switch socket can also be installed on the PCB for easy keyswitch replacement.
+There is a dedicated space in the center of the keyboard for mounting your favorite keycaps, allowing you to display Artisan keycaps or your favorite keycaps.
+In fact, you can even place a rotary encoder there.
+
+* Keyboard Maintainer: [takashicompany](https://github.com/takashicompany)
+* Hardware Supported: GOAT51 PCB, Pro Micro
+* Hardware Availability: https://github.com/takashicompany/goat51
+
+Make example for this keyboard (after setting up your build environment):
+
+    make takashicompany/goat51:default
+
+Flashing example for this keyboard:
+
+    make takashicompany/goat51: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