Update README

README.md

@@ -198,3 +198,33 @@ In the default script of AutoHotkey you can define custom hotkeys.
 
 The hotkeys above are for the combination CtrlAltGui and CtrlAltGuiShift plus the letter a.
 AutoHotkey inserts the Text right of `Send, ` when this combination is pressed.
+
+## RGB Under Glow Mod
+
+![Planck with RGB Underglow](https://raw.githubusercontent.com/yangliu/qmk_firmware/planck-rgb/keyboard/planck/keymaps/yang/planck-with-rgb-underglow.jpg)
+
+Here is a quick demo on Youtube (with NPKC KC60) (https://www.youtube.com/watch?v=VKrpPAHlisY).
+
+For this mod, you need an unused pin wiring to DI of WS2812 strip. After wiring the VCC, GND, and DI, you can enable the underglow in your Makefile.
+
+    RGBLIGHT_ENABLE = yes
+
+Please note that the underglow is not compatible with MIDI functions. So you cannot enable both of them at the same time.
+
+Please add the following options into your config.h, and set them up according your hardware configuration.
+
+    #define ws2812_PORTREG  PORTF
+    #define ws2812_DDRREG   DDRF
+    #define ws2812_pin PF4
+    #define RGBLED_NUM 14     // Number of LEDs
+    #define RGBLIGHT_HUE_STEP 10
+    #define RGBLIGHT_SAT_STEP 17
+    #define RGBLIGHT_VAL_STEP 17
+
+The firmware supports 5 different light effects, and the color (hue, saturation, brightness) can be customized in most effects. To control the underglow, you need to modify your keymap file to assign those functions to some keys/key combinations. For details, please check this keymap. `keyboard/planck/keymaps/yang/keymap.c`
+
+### WS2812 Wiring
+
+![WS2812 Wiring](https://raw.githubusercontent.com/yangliu/qmk_firmware/planck-rgb/keyboard/planck/keymaps/yang/WS2812-wiring.jpg)
+
+Please note the USB port can only supply a limited amount of power to the keyboard (500mA by standard, however, modern computer and most usb hubs can provide 700+mA.). According to the data of NeoPixel from Adafruit, 30 WS2812 LEDs require a 5V 1A power supply, LEDs used in this mod should not more than 20.