I think I've found what I needed in the header of ESP8266Gateway.ino:

 * See http://www.mysensors.org/build/ethernet_gateway for wiring instructions.
 * The ESP8266 however requires different wiring:
 * nRF24L01+  ESP8266
 * VCC        VCC
 * CE         GPIO4          
 * CSN/CS     GPIO15
 * SCK        GPIO14
 * MISO       GPIO12
 * MOSI       GPIO13