added a description how to flash jj40 in windows (#3247)

keyboards/jj40/README.md

@@ -52,6 +52,13 @@ $ make jj40
 $ bootloadHID -r jj40_default.hex
 ```
 
+For Windows 10:
+Windows sometimes doesn't recognize the jj40. The easiest way of flashing a new layout is probably using [HIDBootFlash](http://vusb.wikidot.com/project:hidbootflash).
+1. Go to Windows Device Manager and find the keyboard (plug it in while holding down `Backspace` (`Top Right Key`)). It can be found under Human Interface Devices or under Keyboards.
+2. Go to properties and the Details tab to find the hardware ID. You want the VID and the PID (code after the underscore). Plug them into HIDBootFlash and hit Find Device.
+3. Use `make jj40:<keymap-name>` to generate the .hex file in the qmk basis folder. Select the .hex file in HIDBootFlash and press Flash Device.
+
+
 ## Troubleshooting
 
 1. Try plugging the board in while pressing `Backspace` (`Top Right Key`). This will force it