Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/vendor/guzzlehttp/ringphp/README.rst

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+=======
+RingPHP
+=======
+
+Provides a simple API and specification that abstracts away the details of HTTP
+into a single PHP function. RingPHP be used to power HTTP clients and servers
+through a PHP function that accepts a request hash and returns a response hash
+that is fulfilled using a `promise <https://github.com/reactphp/promise>`_,
+allowing RingPHP to support both synchronous and asynchronous workflows.
+
+By abstracting the implementation details of different HTTP clients and
+servers, RingPHP allows you to utilize pluggable HTTP clients and servers
+without tying your application to a specific implementation.
+
+.. code-block:: php
+
+    <?php
+    require 'vendor/autoload.php';
+
+    use GuzzleHttp\Ring\Client\CurlHandler;
+
+    $handler = new CurlHandler();
+    $response = $handler([
+        'http_method' => 'GET',
+        'uri'         => '/',
+        'headers'     => [
+            'host'  => ['www.google.com'],
+            'x-foo' => ['baz']
+        ]
+    ]);
+
+    $response->then(function (array $response) {
+        echo $response['status'];
+    });
+
+    $response->wait();
+
+RingPHP is inspired by Clojure's `Ring <https://github.com/ring-clojure/ring>`_,
+which, in turn, was inspired by Python's WSGI and Ruby's Rack. RingPHP is
+utilized as the handler layer in `Guzzle <http://guzzlephp.org>`_ 5.0+ to send
+HTTP requests.
+
+Documentation
+-------------
+
+See http://ringphp.readthedocs.org/ for the full online documentation.