Update readme.md

Author: rgodiyal

readme.md

@@ -45,6 +45,7 @@ echo '</pre>';
 die();
 
 ```
+
 Create Shipment | Shipping Label
 ```php
 <?php
@@ -247,4 +248,23 @@ echo '<pre>'; print_r($shipRes); echo '</pre>';
 </html>
 
 ```
+Tracking API
+```php
+<?php
+
+use RahulGodiyal\PhpUpsApiWrapper\Tracking;
+
+require_once('./vendor/autoload.php');
+
+$client_id = "xxxxxxxxxxxxxxxx"; // UPS Client ID
+$client_secret = "xxxxxxxxxxxxxxx"; // UPS Client Secret
+
+$trackingRes = Tracking::setTrackingNumber("123456789")->fetch($client_id, $client_secret); // For Dev Api
+$trackingRes = Tracking::setTrackingNumber("123456789")->setMode('PROD')->fetch($client_id, $client_secret); // For PROD Api
+
+echo '<pre>';
+print_r($trackingRes);
+echo '</pre>';
+die();
+```