Updated Bag of Tricks (markdown)

Bag-of-Tricks.md

@@ -1,5 +1,31 @@
 Here are helper class that can help you add missing features.
 
+## Look for a nested key
+
+Suppose we've this json:
+
+```json
+{"key1":1,"key2":{"subkey1":3,"subkey2":4}}
+```
+
+And we want to check presence of **subkey1** (or **subkey2**). Using **containsKey** method will not work, since **subkey1** and **subkey2** are nested keys inside **key2** value. This function allows to find key presence even if they are nested inside any key value:
+
+```c++
+bool containsNestedKey(const JsonObject& obj, const char* key) {
+    for (const JsonPair& pair : obj) {
+        if (!strcmp(pair.key, key))
+            return true;
+
+        if (containsNestedKey(pair.value.as<JsonObject>(), key)) 
+            return true;
+    }
+
+    return false;
+}
+```
+
+See issue [#322](https://github.com/bblanchon/ArduinoJson/issues/322)
+
 ## Extract an array of integer
 
 Suppose we have an input like this: