[Keyboard] Add supersplit (#18611)

Co-authored-by: tarneo <tarneo@tarneo.fr>

keyboards/supersplit/readme.md

@@ -0,0 +1,25 @@
+# supersplit
+
+*A custom-made keyboard with a 4x6 matrix and a 9-key thumb cluster*
+
+* Keyboard Maintainer: [tarneaux](https://github.com/tarneaux)
+* Hardware Supported: elite-c
+* Hardware Availability: https://github.com/tarneaux/supersplit
+
+Make example for this keyboard (after setting up your build environment):
+
+    make supersplit:default
+
+Flashing example for this keyboard:
+
+    make supersplit: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