Updated FAQ (markdown)

FAQ.md

@@ -137,3 +137,30 @@ void serialize(const SensorData& data, char* json, size_t maxSize)
 As you can see the `StaticJsonBuffer` is kept in memory as short as possible, so that the remain of your program is unaffected by the JSON serialization.
 
 Also you can see that neither `JsonArray` nor `JsonObject` leak out of the serialization code. This maintain a good isolation and reduce the coupling with the library.
+
+# How to fix error "Ambiguous overload for 'operator='"
+
+There is a case where you need the compiler: it's when you convert a `JsonVariant` to a `String`.
+
+For example:
+
+```c++
+String ssid = network["ssid"];
+ssid = network["ssid"];
+```
+
+The first line will compile but the second will fail with the following error:
+
+```
+error: ambiguous overload for 'operator=' (operand types are 'String' and 'ArduinoJson::JsonObjectSubscript<const char*>')
+```
+
+The solution is to remove the ambiguity with any of the following replacement:
+
+```c++
+ssid = (String)network["ssid"];
+ssid = (const char*)network["ssid"];
+ssid = network["ssid"].asString();
+ssid = network["ssid"].as<const char*>();
+ssid = network["ssid"].as<String>();
+```