Added links to the book page in each example

examples/JsonConfigFile/JsonConfigFile.ino

@@ -10,6 +10,8 @@
 //   "hostname": "examples.com",
 //   "port": 2731
 // }
+//
+// https://arduinojson.org/v6/example/config/
 
 #include <ArduinoJson.h>
 #include <SD.h>
@@ -130,4 +132,15 @@ void loop() {
   // not used in this example
 }
 
-// Visit https://arduinojson.org/v6/example/config/ for more.
+// See also
+// --------
+//
+// https://arduinojson.org/ contains the documentation for all the functions
+// used above. It also includes an FAQ that will help you solve any
+// serialization or deserialization problem.
+//
+// The book "Mastering ArduinoJson" contains a case study of a project that has
+// a complex configuration with nested members.
+// Contrary to this example, the project in the book uses the SPIFFS filesystem.
+// Learn more at https://arduinojson.org/book/
+// Use the coupon code TWENTY for a 20% discount ❤❤❤❤❤