[Keyboard] Add Lain keyboard (#16345)

Co-authored-by: Joel Challis <git@zvecr.com>
Co-authored-by: Ryan <fauxpark@gmail.com>

keyboards/dm9records/lain/readme.md

@@ -0,0 +1,32 @@
+# lain
+
+![lain](https://i.imgur.com/V0mR1Bgh.jpg)
+
+Ergonomic 40% keyboard.
+
+serial exprement lain themed.    
+licensed under #lainttl (https://www.nbcuni.co.jp/rondorobe/anime/lain/ttl/)
+
+* Keyboard Maintainer: [Takuya Urakawa @hsgw](https://github.com/hsgw), https://5z6p.com
+* Hardware Supported: onboard MCU.
+* Hardware Availability: Group buy. (https://5z6p.com/products/lain/)
+
+Make example for this keyboard (after setting up your build environment):
+
+    make dm9records/lain: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).
+
+## Note
+### PCB revision
+
+If your pcb is not Rev.1, you should use the firmware in below repository.   
+https://github.com/hsgw/qmk_firmware/tree/lain/keyboards/dm9records/lain
+
+### 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
+* **Keycode in layout**: Press the key mapped to `RESET` if it is available