[Keyboard] Add Borderland (#15501)

Co-authored-by: Drashna Jaelre <drashna@live.com>

keyboards/kiwikey/borderland/readme.md

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+# Borderland
+
+![Borderland](https://i.imgur.com/3aMyUJb.jpg)
+
+Another keyboard with well-known Arisu layout, comes with USB Type-C connection and Rotary Encoder. More information can be found at https://kiwikey.vn/borderland/
+
+* Keyboard Maintainer: [KiwiKey](https://github.com/kiwikey)
+* Hardware Supported: Borderland PCB rev1
+* Hardware Availability: [KiwiKey Website](https://kiwikey.vn/)
+
+Make example for this keyboard (after setting up your build environment):
+
+    make kiwikey/borderland:default
+
+Flashing example for this keyboard:
+
+    make kiwikey/borderland: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
+* **Keycode in layout**: Press the key mapped to `RESET`, in default keymap it is `Fn+R`