Updated FAQ (markdown)

FAQ.md

@@ -129,6 +129,12 @@ This LED can be very distracting. Unfortunately it can't easily be disabled as i
 - Remove the LED permanently (desolder or apply pressure with e.g. a flathead screwdriver)
 - Use a different `LEDPIN`, although the default is recommended for stable operation
 </details>
+
+<details>
+<summary>Why is gpio2/D4 the default LED pin?</summary>
+
+Although pins D1 and D2 are usually regarded the best GPIO pins to use in an ESP8266 project, D4 is the default in WLED, despite having two major and one minor drawback. The major drawbacks are the permanently lit blue onboard LEDs and the fact that the pin level needs to be high (pullup) at powerup or the controller will not boot. A minor drawback is that the `Serial1` bus can not be used, but this is irrelevant in most cases, as the USB/serial converter is connected to the other `Serial` interface. The reason for using this pin is that it uses UART hardware driving, which increases stability and decreases CPU overhead especially with larger amounts of pixels.
+</details>
    
 ## General Questions