Add elephant42 keyboard (#14336)

keyboards/elephant42/readme.md

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+# elephant42
+
+[![elephant42](https://i.imgur.com/rgvNteb.jpg)](https://i.imgur.com/rgvNteb.jpg)
+
+A split keyboard with 3x5+2 vertically staggered keys and 4 thumb keys.
+
+* Keyboard Maintainer: [illness072](https://github.com/illness072), [Twitter](https://twitter.com/illness072)
+* Hardware Supported: elephant42 PCB, ProMicro
+* Hardware Availability: [https://illness072.booth.pm/](https://illness072.booth.pm/)  
+
+Make example for this keyboard (after setting up your build environment):
+
+    make elephant42:default
+
+Flashing example for this keyboard:
+
+    make elephant42: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 `RESET` if it is available