Prepare ymdk/ymd75 for rev4 (#21484)

keyboards/ymdk/ymd75/rev1/readme.md

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+# YMD75 / MT84
+
+75% keyboard from YMDK. YMDK sell the board and name it "YMD75", however revision 1 of the PCB has "MT84" printed on both sides.
+
+This firmware can also be flashed onto the KBDFans KBD75 Round 2 Bootmapper Client boards sold during the mid-2018 timeframe.
+
+* Keyboard Maintainer: [Wayne K Jones](github.com/WarmCatUK)
+* Hardware Supported: rev1 (ATmega32a)
+* Hardware Availability: <https://www.aliexpress.com/item/32812690592.html>
+
+Make example for this keyboard (after setting up your build environment):
+
+    make ymdk/ymd75/rev1:default
+
+Flashing example for this keyboard ([after setting up the bootloadHID flashing environment](https://docs.qmk.fm/#/flashing_bootloadhid))
+
+    make ymdk/ymd75/rev1: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
+* **Reset Key**: Hold down the key located at *Key below the top right key*, commonly programmed as *Backspace* while plugging in the keyboard.
+* **Keycode in layout**: Press the key mapped to `QK_BOOT` if it is available