Added readme

Author: Nyholm

README.md

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+# Guzzle test server
+
+This server has long been part of the Guzzle Http client. Since late 2021 it
+was extracted to its own package.
+
+The server is useful to use in your integration tests.
+
+```php
+
+use GuzzleHttp\Server\Server;
+
+Server::start();
+register_shutdown_function(static function () {
+    Server::stop();
+});
+
+Server::enqueue([
+    new Response(201),
+]);
+
+$myHttpClient = MyClient();
+$response = $this->makeRequest('GET', Sever::$url);
+// $response will be 201
+
+$requests = Server::received();
+// $request[0] is the one sent by MyClient
+
+```