[Keyboard] add ydpm40 keyboard (#19294)

keyboards/ydkb/ydpm40/readme.md

@@ -0,0 +1,22 @@
+# ydkb/ydpm40
+
+A pair of 40% keyboards, that share the same PCB, designed by yang. Among the two, ydp40 has a Planck-like layout, while ydm40 is more traditional.
+
+* Keyboard Maintainer: [xofyarg](https://github.com/xofyarg)
+* Hardware Supported: https://github.com/yangdigi/YDPM40-Keyboard-PCB
+* Hardware Availability: https://ydkb.world.taobao.com/
+
+Make example for this keyboard (after setting up your build environment):
+
+    make ydkb/ydpm40:default
+
+Flashing example for this keyboard:
+
+    make ydkb/ydpm40: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
+
+The ATmega328P MCU comes with BootloadHID. To enter it, hold down the
+top left key and plug in the keyboard.