Update build instruction

Author: datxuantran

README.md

@@ -72,7 +72,9 @@ Grab the toolchain from https://github.com/espressif/ESP8266_RTOS_SDK#developing
 
 ```bash
 git clone --recursive https://github.com/walmis/blackmagic-espidf.git
-cd blackmagic-espidf
+cd blackmagic-espidf/ESP8266_RTOS_SDK
+git checkout master # checkout for lastest patches of ESP8266_RTOS_SDK
+cd ../
 make menuconfig # optional, if you want to change some settings
 make
 make flash # this will flash using esptool.py over serial connection