[Keymap] Add initial Unicorne keyboard layout for bcat (#15613)

keyboards/yanghu/unicorne/keymaps/bcat/readme.md

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+# bcat's Unicorne layout
+
+This ergo layout follows my preferred [split 3x6 + 3 thumb
+keys](https://github.com/qmk/qmk_firmware/tree/master/layouts/community/split_3x6_3/bcat)
+layout, adding bindings for the two rotary encoders on the bottom row. (The
+left encoder scrolls vertically and the right encoder adjusts volume.) Also,
+since the Unicorne keyboard has a speaker, I added bindings on the adjust layer
+to activate and configure music mode. Finally, an adorable unicorn prances on
+the OLED, running faster the quicker you type.
+
+## Default layer
+
+![Default layer layout](https://i.imgur.com/Max5R0T.png)
+
+([KLE](http://www.keyboard-layout-editor.com/#/gists/b6d0b16a913e7d1faeafc9fc751c413f))
+
+## Lower layer
+
+![Lower layer layout](https://i.imgur.com/E8Cfowc.png)
+
+([KLE](http://www.keyboard-layout-editor.com/#/gists/99dd65d3b857a272be7a1804b20bc266))
+
+## Raise layer
+
+![Raise layer layout](https://i.imgur.com/KomdMyB.png)
+
+([KLE](http://www.keyboard-layout-editor.com/#/gists/cf9e899867763dc45b65917ce4cf93ff))
+
+## Adjust layer
+
+![Adjust layer layout](https://i.imgur.com/EtshZbn.png)
+
+([KLE](http://www.keyboard-layout-editor.com/#/gists/7eb0f1c437169f30cc18eac271ad2302))