Updated README.md

Updated README.md with preserve authentication token example.

README.md

@@ -488,6 +488,42 @@ print_r($message);
 ?>
 ```
 
+### Send a single SMS while preserving authentication token (for multiple concurrent executions)
+
+```php
+<?php
+
+//Your account username and password
+$username = "";
+$password = "";
+
+//Preserve an access_token if it already exists and it is not expired. Used when multiple nodes use the same account to access the API.
+$preserve=true;
+
+//Endpoint Options:
+//  APIClient::API_ENDPOINT - REST API (SMS, OTP, HLR) - Default
+//  APIClient::SMPP_API_1_ENDPOINT - Asynchronous SMS API (SMS)
+//  APIClient::SMPP_API_2_ENDPOINT - Asynchronous SMS API (SMS) - Alternative
+$endpoint = MobiWeb\Rest\Client::SMPP_API_1_ENDPOINT;
+
+$client = new MobiWeb\Rest\Client($username, $password, $endpoint, $preserve);
+
+//Submit message
+$message = $client->broadcast(
+    [[
+      "from" => "HelloWorld", //The sender displayed upon the SMS arrival. Can be composed of 2-11 alphanumeric characters (A-z,0-9, ,-,.) or 14 numeric characters (0-9). Special characters are not allowed.
+      "to" => ["44xxxxxxxxxx"], //The full international number(s) of the recipient(s) in international E.164 format https://en.wikipedia.org/wiki/E.164.
+      "message" => "Hello from MobiWeb!" //The text of the SMS message. If all characters in the message belong to the 3GPP GSM 7-bit GSM 03.38 ASCII character table https://en.wikipedia.org/wiki/GSM_03.38#GSM_7-bit_default_alphabet_and_extension_table_of_3GPP_TS_23.038_/_GSM_03.38, you can send up to 160 characters in a single SMS. You can send longer messages automatically by setting message parameter with your preffered text. In long SMS (multi-part SMS), each SMS part can be up to 153 characters. Each part costs as 1 SMS. If one or more characters in the message belong to the 16-bit Unicode / UCS-2 character table https://en.wikipedia.org/wiki/UTF-16, because of the increased memory requirement for each character, you can send up to 70 characters in a single SMS. In long SMS (multi-part SMS), each SMS part can be up to 67 characters. Each part costs as 1 SMS.
+
+    ]]
+  );
+
+//Print message
+print_r($message);
+
+?>
+```
+
 ### Send multiple SMS
 
 ```php