Added example for callback (webhook) script

Read more about callbacks:
https://docs.qencode.com/navigate/index/010#010_080

Author: qencode-dev

examples/callback_example.php

@@ -0,0 +1,17 @@
+<?php
+/**
+ * Receives a callback from Qencode when a job is completed.
+ * Prints out job status and output video URL(s)
+ */
+
+$logfile = date('Y-m-d_H-i-s').'.log';
+$job_info = json_decode($_POST['status']);
+$log = "Job status: {$job_info->status}\n";
+if ($job_info->status == 'completed') {
+    foreach ($job_info->videos as $video) {
+        $log .= "URL: {$video->url}\n";
+    }
+
+}
+print $log;
+file_put_contents($logfile, $log);