Added "Do not assume that strings are copied"

Avoiding pitfalls.md

@@ -70,7 +70,47 @@ For instance, let's imagine that you parse `["hello","world"]`, like this:
 In that case, both `first` and `second` are pointers to the content of the original string `json`.
 So this will only work if `json` is still in memory.
 
-#### 6. Make sure the string isn't read-only
+#### 6. Do not assume that strings are copied
+
+By design ArduinoJson **never makes copies** of strings.
+This allows it to work with full static memory allocation and ensure an efficient use of CPU cycles.
+
+But this can have subtle consequences...
+
+For instance the following code will not work as expected:
+
+    JsonArray& array = jsonBuffer.createArray();
+    for (int i=0; i<3; i++) {
+        char buffer[16];
+        sprintf(buffer, "iteration %d", 0);
+        array.add(buffer);
+    }
+    array.printTo(Serial);
+
+One will probably expect the following result:
+
+    ["iteration 0","iteration 1","iteration 2"]
+
+but the actual result would be:
+
+    ["iteration 2","iteration 2","iteration 2"]
+
+because the same memory area has been reuse for each iteration.
+
+The solution is to make sure that each string is available in memory by the time `printTo()` is called.
+For instance, one could allocate all strings out of the loop:
+
+    JsonArray& array = jsonBuffer.createArray();
+    char buffer[16][3];
+    for (int i=0; i<3; i++) {
+        sprintf(buffer[i], "iteration %d", 0);
+        array.add(buffer[i]);
+    }
+    array.printTo(Serial);
+
+The same principle applies to key and values of `JsonObject`.
+
+#### 7. Make sure the string isn't read-only
 
 If you read carefully the previous section, you may have come to the conclusion that the JSON parser modifies the JSON string.
 
@@ -96,4 +136,3 @@ If you replace it by:
 Depending on your platform, you may have an exception because the parser tries to write at a location that is read-only.
 In the first case `char json[]` declares an array of `char` initialized to the specified string.
 In the second case `char* json` declares a pointer to a read-only string, in fact it should be a `const char*` instead of a `char*`.
-