add usage example with qr generaotr

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "phpqrcode"]
+	path = phpqrcode
+	url = https://github.com/t0k4rt/phpqrcode

example.php

@@ -0,0 +1,11 @@
+<?php
+require 'libpromptpay.php';
+require 'phpqrcode/qrlib.php';
+
+$pp = new LibPromptpay();
+$pp->setPaymentID(LibPromptpay::PROMPTPAY_ACCTYPE_EWALLET, '004999000281286');
+$pp->setQRType(LibPromptpay::EMVCO_QRTYPE_DYNAMIC);
+$pp->setAmount(1);
+
+header('Content-Type: image/png');
+QRcode::png($pp->generateString());