[Keyboard] add keyboard selka40 (#14560)

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

keyboards/dyz/selka40/readme.md

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+# selka40
+
+![pcb](https://i.imgur.com/pqnEVngh.jpg)
+
+40% (13u) ergo hotswap with multiple layouts for bottom row
+
+-   Keyboard Maintainer: [dayatz](https://github.com/dayatz)
+-   Hardware Supported: ATMega32u4
+
+Make example for this keyboard (after setting up your build environment):
+
+    make dyz/selka40:default
+
+Flashing example for this keyboard:
+
+    make dyz/selka40: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 the `Esc` key while plugging in the USB cable
+-   **Physical reset button**: Briefly press the button on the back of the PCB
+-   **Keycode in layout**: Press the key mapped to `RESET` if it is available