[Keyboard] Add a new keyboard 'plum47' (#18359)

keyboards/plum47/readme.md

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+# plum47
+
+![plum47](https://i.imgur.com/t3rjsWIh.png)
+
+Plum47 is a 4-row, 12-column (47 key) ortholinear keyboard sold domestically in Japan as a 
+home build kit.  It features MX sockets for hot-swap keyswitch support, and a bezel-less
+acrylic sandwich case.  The controller is a Pro Micro.
+
+* Keyboard Developer: [Masayuki Takagi](https://github.com/takagi)
+* Keyboard submitted by [Andrew Hamilton](https://github.com/awh-tokyo)
+* Hardware Supported: plum47 keyboard kit including PCB and Pro Micro controller
+* Hardware Availability: https://shop.yushakobo.jp/products/consign_plum47
+
+Make example for this keyboard (after setting up your build environment):
+
+    make plum47:default
+
+Flashing example for this keyboard:
+
+    make plum47: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 as follows:
+
+* **Bootmagic reset**: Hold down the key at (0,0) in the matrix (usually the top left key or Escape) and plug in the keyboard