Add setCookie example

examples/set_cookie.php

@@ -0,0 +1,9 @@
+<?php
+require __DIR__ . '/vendor/autoload.php';
+
+use \Curl\Curl;
+
+$curl = new Curl();
+$curl->setCookie('foo', 'bar');
+$curl->get('https://httpbin.org/cookies');
+var_dump($curl->response->cookies->foo === 'bar');