Updated FAQ (markdown)

FAQ.md

@@ -58,6 +58,55 @@ for (int i=0; i<10; i++) {
 
 Note that, contrary to a common belief, moving a `StaticJsonBuffer` inside of a loop has no negative impact on performance.
 
+##### Cause 2: reuse of JSON input
+
+In order to make the JSON parsing without any allocation or duplication, ArduinoJson modifies the string in place: it inserts null terminators and unescapes special characters.
+
+If you provide a writeable input, like a `char[]` or a `char*`, it will modify this input in place.
+If you provide a read only input, like a `const char*` or a `String`, it will have to make a copy of it in order to be allowed to modify it.
+
+That's why it's highly recommended to used a writeable input: you get a huge performance boost and memory usage is greatly reduced :+1:
+
+Now, this behavior leads to unexpected result if you try to reuse the modified string, for instance:
+
+```c++
+char json[256];
+readJsonFromSomewhere(json, sizeof(json));
+
+for (int i=0; i<10; i++) {
+    StaticJsonBuffer<200> jsonBuffer;
+
+    JsonObject& root = jsonBuffer.parse(json);
+    if (root.success()) {
+       Serial.println("parseObject() succeeded");
+    } else {
+       Serial.println("parseObject() failed!");
+    }
+}
+```
+
+Only the first call to `parseObject()` will succeed because after that call, `json` will be altered and not be valid JSON anymore.
+
+The solution is simply to parse the input only once, or get a fresh input at each iteration:
+
+```c++
+for (int i=0; i<10; i++) {
+    char json[256];
+    readJsonFromSomewhere(json, sizeof(json));
+
+    StaticJsonBuffer<200> jsonBuffer;
+
+    JsonObject& root = jsonBuffer.parse(json);
+    if (root.success()) {
+       Serial.println("parseObject() succeeded");
+    } else {
+       Serial.println("parseObject() failed!");
+    }
+}
+```
+
+See issue [#153](https://github.com/bblanchon/ArduinoJson/issues/153)
+
 ### What are the differences between `StaticJsonBuffer` and `DynamicJsonBuffer`?
 
                  | `StaticJsonBuffer`  | `DynamicJsonBuffer`