Add note about recent 3.0.0 update

README.md

 esp8266-oled-ssd1306 [![Build Status](https://travis-ci.org/squix78/esp8266-oled-ssd1306.svg?branch=dev-branch-3.0.0)](https://travis-ci.org/squix78/esp8266-oled-ssd1306)
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+> We just released version 3.0.0. Please have at our [upgrade guide](UPGRADE-3.0.md)
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 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