Added comment to remind me the reason for a random bit of code

Author: jeremypoulter

src/hal.cpp

@@ -51,6 +51,7 @@ void HalClass::begin()
   DEBUG_BEGIN(115200);
 
 #ifdef SERIAL_RX_PULLUP_PIN
+  // https://forums.adafruit.com/viewtopic.php?f=57&t=153553&p=759890&hilit=esp32+serial+pullup#p769168
   pinMode(SERIAL_RX_PULLUP_PIN, INPUT_PULLUP);
 #endif