GPIO2 note updated

Updated note on GPIO2 with informations about possible pull-down resistor.

Merges espressif/esp-idf#1451

Author: Pomezny

examples/storage/sd_card/README.md

@@ -37,6 +37,8 @@ With the given pinout for SPI mode, same connections between the SD card and ESP
 
 GPIO2 pin is used as a bootstrapping pin, and should be low to enter UART download mode. One way to do this is to connect GPIO0 and GPIO2 using a jumper, and then the auto-reset circuit on most development boards will pull GPIO2 low along with GPIO0, when entering download mode.
 
+- Some boards have pulldown and/or LED on GPIO2. LED is usually ok, but pulldown will interfere with D0 signals and must be removed. Check the schematic of your development board for anything connected to GPIO2. 
+
 ### Note about GPIO12
 
 GPIO12 is used as a bootstrapping pin to select output voltage of an internal regulator which powers the flash chip (VDD_SDIO). This pin has an internal pulldown so if left unconnected it will read low at reset (selecting default 3.3V operation). When adding a pullup to this pin for SD card operation, consider the following: