add example for using configuration's parenthesis operator

src/Configuration.hpp

@@ -100,6 +100,17 @@ public:
      * Get a read-only reference to the value at the given hierarchy of keys. If no type is specified for the return type, a
      * JSON object will be returned.
      *
+     * Example,
+     *
+     *     std::cout << _configuration() << std::endl;
+     *     std::cout << _configuration("recording", "enabled") << " " << _configuration("input") << " " << _configuration("levels", 0, 3, 1) << std::endl;
+     *
+     * Prints,
+     *
+     *     [full config...]
+     *     true {"any-key-ignore-commands":[],"default-unsuppress-delay":0.7,"ignore-repeat-keypress":true,"suppress-any-key-on-mods":true, \
+     *         "system-any-key-ignore-commands":["fullscreen","screenshot","record","quit"]} 261.0
+     *         
      * @param keys   hierarchy of keys used to look up a specific value in the config JSON
      * @return       read-only reference to value specified by keys
      */