Add upgrade guide

README.md

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-# ESP8266 OLED SSD1306 
+# ESP8266 OLED SSD1306
 
 > We just released version 3.0.0. Please have a look at our [upgrade guide](UPGRADE-3.0.md)
 
@@ -26,6 +26,8 @@ Check out the examples folder for a few comprehensive demonstrations how to use
 
 The API changed a lot with the 3.0 release. If you were using this library with older versions please have a look at the [Upgrade Guide](UPGRADE-3.0.md).
 
+Going from 3.x version to 4.0 a lot of internals changed and compatibility for more displays was added. Please read the [Upgrade Guide](UPGRADE-4.0.md)
+
 ## Features
 
 * Draw pixels at given coordinates

UPGRADE-4.0.md

+# Upgrade from 3.x to 4.0
+
+There is one change that breaks compatibility with older versions. You'll have to change data type for all your binary resources such as images and fonts from
+
+```c
+const char MySymbol[] PROGMEM = {
+```
+
+to
+
+```c
+const uint8_t MySymbol[] PROGMEM = {
+```