[Keyboard] Jj40 doc (#4914)

* Update README.md

* Update README.md

* Update README.md

* Update README.md

keyboards/jj40/README.md

@@ -69,3 +69,16 @@ Windows sometimes doesn't recognize the jj40. The easiest way of flashing a new
 3. If you get an error such as "Resource Unavailable" when attemting to flash
    on Linux, you may want to compile and run `tools/usb_detach.c`. See `tools/README.md`
    for more info.
+
+## Recovery 
+If you flash a bad hex (e.g. you have a V1 board without RGB and compile/flash blindly without editing your rules.mk), your jj40 is now semi-bricked and you're stuck unless you have access to an ISP. The [ISP Flashing Guide](https://docs.qmk.fm/#/isp_flashing_guide) contains very good (but somewhat generalized) information. However, the instructions below should get you up and running provided you have an Arduino or clone.
+
+### Arduino Setup 
+1. Upload the ArduinoISP sketch onto your Arduino board (https://www.arduino.cc/en/Tutorial/ArduinoISP).  
+2. Wire the Arduino to the jj40. Match the data pins on the Arduino to those on the jj40. "RST" usually goes to D10 on the Arduino. I didn't need a capacitor when using my Uno. 
+![Imgur](https://i.imgur.com/oLWJOkQ.jpg)
+3. Get a working bootloader from https://blog.winkeyless.kr/m/152. The file is called "main.hex" from the archive called "ps2avrGB_bootloader_161215.zip"  Copy "main.hex" to your qmk folder.  
+4. Burn the bootloader with the following command  
+` avrdude -b 19200 -c avrisp -p atmega32 -v -e -U hfuse:w:0xD0:m -U lfuse:w:0x0F:m -U flash:w:main.hex:i -P comPORT`  
+Change `comPORT` to whatever port is used by the Arduino (e.g. `com11` in Windows or `/dev/ttyACM0` in Linux). Use Device Manager in Windows to find the port being used. Use `ls /dev/tty*` in Linux.
+5. If this process is successful, you should now be able to upload normally.