[Keyboard] Sejin EAT-1010R2 keyboard (#19809)

Co-authored-by: Joel Challis <git@zvecr.com>
Co-authored-by: Drashna Jaelre <drashna@live.com>
Co-authored-by: jack <0x6a73@protonmail.com>

keyboards/handwired/sejin_eat1010r2/readme.md

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+# QMK-based firmware for Sejin EAT-1010R2 keyboard
+
+* Keyboard Maintainer: [DmNosachev](https://github.com/DmNosachev)
+* Hardware Supported: Sejin EAT-1010R2 (membrane keyboard, Sejin
+Futaba-mount dome, Asian 101 ISO layout), Blue/Black Pill STM32F103 MCU 
+oard. Alternatevely you can use any MCU which is supported by QMK and
+has 27 or more IO pins
+
+Make example for this keyboard (after setting up your build environment):
+
+    make handwired/sejin_eat1010r2:default
+    
+Flashing example for this keyboard:
+
+    make handwired/sejin_eat1010r2: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).
+
+## Additional hardware
+* 16 and 8 pin 2.54mm (0.1'') FPC connectors. They can be found at
+Aliexpress for 2–3 USD for 10pcs. Two 8 pin connectors can be glued
+together as a replacement for 16 pin.
+* Three 5mm LEDs and current limiting resistors (150–300Ω, depending on
+LED specs). This keyboard has exactly the same spacing between LEDs as
+IBM Model M, so a corresponding PCB can be used after adding resistors.
+
+## Membrane
+Sejin EAT-1010R2 has 16x8 matrix. There are 16 and 8 pin flex connectors
+from membrane sheets. Solder FPC connectors to devboard (numbering from
+left to right, see *matrix_pins* in info.json file).
+
+## Bootloader
+Burn [STM32duino bootloader](https://github.com/rogerclarkmelbourne/STM32duino-bootloader)
+to Blue Pill board.
+
+There are several ways to get into the bootloader:
+* STM32duino bootloader waits for 3 seconds in DFU mode before jumping to application.
+You just have to start the flashing process, then connect the USB.
+* This firmware is configured with [bootmagic feature](https://docs.qmk.fm/#/feature_bootmagic).
+Hold the ESC key down when plugging the keyboard in to trigger the bootloader.
+
+## Troubleshooting
+There is *debug* layout which has mapping for every matrix position and
+prints column and raw numbers to console (hid_listen or QMK toolbox).