Updated Compatibility issues (markdown)

Compatibility-issues.md

@@ -6,6 +6,7 @@
 - [Warning: ignoring packed attribute because of unpacked non-POD field](#warning-ignoring-packed-attribute-because-of-unpacked-non-pod-field)
 - [Error: undefined reference to `__cxa_guard_acquire` and `__cxa_guard_release`](#error-undefined-reference-to-cxaguardacquire-and-cxaguardrelease)
 - [Adafruit WICED](#adafruit-wiced)
+- [ESP32](#esp32)
 
 <!-- /MarkdownTOC -->
 
@@ -78,3 +79,20 @@ error: cannot convert 'err_t' to 'err_t (*)()
 ```
 
 See issue [#404](https://github.com/bblanchon/ArduinoJson/issues/404)
+
+## ESP32
+
+There is currently a bug in the [Arduino Core for ESP32](https://github.com/espressif/arduino-esp32) causing the following error:
+
+```
+error: redefinition of 'struct ArduinoJson::Internals::StringFuncs<const char*>'
+```
+
+The solution is to disable `PROGMEM` support in ArduinoJson.
+To do that, just add the following line at the top of your program:
+
+```c++
+#define ARDUINOJSON_ENABLE_PROGMEM 0
+```
+
+See issue [#407](https://github.com/bblanchon/ArduinoJson/issues/407)