Added SPI specific comments and SPI display image

README.md

 # esp8266-oled-ssd1306
 
 This is a driver for the SSD1306 based 128x64 pixel OLED display running on the Arduino/ESP8266 platform.
+Can be used with either the I2C or SPI version of the display
 
 You can either download this library as a zip file and unpack it to your Arduino/libraries folder or (once it has been added) choose it from the Arduino library manager.
 
 ## Credits
 Many thanks go to Fabrice Weinberg (@FWeinb) for optimizing and refactoring the UI library.
 The init sequence for the SSD1306 was inspired by Adafruits library for the same display.
+The SPI code was inspired by somhi/ESP_SSD1306 and the Adafruit library
 
 ## Usage
 
@@ -187,3 +189,9 @@ This frame demonstrates the text alignment. The coordinates in the frame show re
 ![DemoFrame4](https://github.com/squix78/esp8266-oled-ssd1306/raw/master/resources/DemoFrame4.jpg)
 
 This shows how to use define a maximum width after which the driver automatically wraps a word to the next line. This comes in very handy if you have longer texts to display.
+
+### SPI version
+
+![SPIVersion](https://github.com/neptune2/esp8266-oled-ssd1306/raw/master/resources/SPI_version.jpg)
+
+This shows the code working on the SPI version of the display. See demo code for ESP8266 pins used.