Added links to the book page in each example

examples/ProgmemExample/ProgmemExample.ino

@@ -8,6 +8,8 @@
 // Use Flash strings sparingly, because ArduinoJson duplicates them in the
 // JsonDocument. Prefer plain old char*, as they are more efficient in term of
 // code size, speed, and memory usage.
+//
+// https://arduinojson.org/v6/example/progmem/
 
 #include <ArduinoJson.h>
 
@@ -56,4 +58,15 @@ void loop() {
   // not used in this example
 }
 
-// Visit https://arduinojson.org/v6/example/progmem/ 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 memory
+// problem.
+//
+// The book "Mastering ArduinoJson" contains a quick C++ course that explains
+// how your microcontroller stores strings in memory. It also tells why you
+// should not abuse Flash strings with ArduinoJson.
+// Learn more at https://arduinojson.org/book/
+// Use the coupon code TWENTY for a 20% discount ❤❤❤❤❤