[Keyboard] Add the KJ-Modify RS40 PCB (#20243)

keyboards/kj_modify/rs40/readme.md

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+# RS40
+
+A compact 40% keyboard with 42 keys.
+
+* Keyboard Maintainer: [Audite Marlow](https://github.com/auditemarlow)
+* Hardware Supported: RS40 PCB
+* Hardware Availability: https://www.aliexpress.com/store/911983021
+
+Make example for this keyboard (after setting up your build environment):
+
+    make kj_modify/rs40:default
+
+Flashing example for this keyboard:
+
+    make kj_modify/rs40: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