Put a note to use the git version of esp32 core.

The release 1.0.4 is on IDF3.2 so the library will not work.

README.md

@@ -9,7 +9,8 @@
 
 ### Installation
 
-- Make sure you use one of the supported versions of Arduino IDE and have ESP32 core installed. (tested on 1.0.4)
+- Make sure you use one of the supported versions of Arduino IDE and have ESP32 core installed
+ <span style="color:red">**Tested only with commit b92c58d of arduino-esp32 core (IDF 3.3). The release 1.0.4 is on an older IDF !**</span>
 - Copy <b>LITTLEFS</b> folder to Arduino IDE embedded libraries place
 - For Win, the default place of arduino-esp32 core libraries is somewhere like: 
 ```C:\Users\<username>\AppData\Local\Arduino15\packages\esp32\hardware\esp32\1.0.4\libraries ```