Add comment about the demo utilizing the Adafruit GPS featherwing

Author: M65649

examples/gps_tracker/gps_tracker.ino

@@ -1,3 +1,8 @@
+/**
+ * This demo utilizes the Adafruit GPS Featherwing to send the GPS position of
+ * the device to a server with HTTP.
+ */
+
 #include <Adafruit_GPS.h>
 #include <Arduino.h>
 #include <http_client.h>