Add guzzle 5 documentation

Author: Márk Sági-Kazár

clients/guzzle5-adapter.rst

@@ -1,2 +1,46 @@
 Guzzle5 Adapter
 ===============
+
+An HTTPlug adapter for the `Guzzle 5 HTTP client`_.
+
+To install the Guzzle adapter, which will also install Guzzle itself (if it was
+not yet included in your project), run:
+
+.. code-block:: bash
+
+    $ composer require php-http/guzzle5-adapter
+
+.. include:: includes/install-message-factory.inc
+
+.. include:: includes/install-discovery.inc
+
+Then begin by creating a Guzzle client, passing any configuration parameters you
+like::
+
+    use GuzzleHttp\Client as GuzzleClient;
+
+    $config = [
+        // Config params
+    ];
+    $guzzle = new GuzzleClientClient($config);
+
+Then create the adapter::
+
+    use Http\Adapter\Guzzle5\Client as GuzzleAdapter;
+    use Http\Message\MessageFactory\GuzzleMessageFactory;
+
+    $adapter = new GuzzleAdapter($guzzle, new GuzzleMessageFactory());
+
+Or if you installed the :ref:`discovery <discovery>` layer::
+
+    use Http\Adapter\Guzzle5\Client as GuzzleAdapter;
+
+    $adapter = new GuzzleAdapter($guzzle);
+
+.. warning::
+
+    The message factory parameter is mandatory if the discovery layer is not installed.
+
+.. include:: includes/further-reading-sync.inc
+
+.. _Guzzle 5 HTTP client: http://docs.guzzlephp.org/en/5.3/

clients/includes/install-discovery.inc

@@ -0,0 +1,6 @@
+Alternatively you can install the :ref:`discovery <discovery>` layer
+to be able to automatically find installed resources, like factories:
+
+.. code-block:: bash
+
+    $ composer require php-http/discovery

clients/includes/install-message-factory.inc

@@ -0,0 +1,18 @@
+This client does not come with a PSR-7 implementation out of the box, so you have
+to install one as well (for example `Guzzle PSR-7`_):
+
+.. code-block:: bash
+
+$ composer require guzzlehttp/psr7
+
+In order to provide full interoperability, message implementations are
+accessed through :ref:`factories <message-factory>`. Message factories for
+`Diactoros`_ are `Guzzle PSR-7`_ implementations are available in the
+:ref:`message <message>` component:
+
+.. code-block:: bash
+
+$ composer require php-http/message
+
+.. _Diactoros: http://docs.guzzlephp.org/en/5.3/
+.. _Guzzle PSR-7: https://github.com/guzzle/psr7