minor #5512 [Cookbook] Backport PSR-7 bridge docs to 2.3 (dunglas, weaverryan)

This PR was merged into the 2.3 branch.

Discussion
----------

[Cookbook] Backport PSR-7 bridge docs to 2.3

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

I think we made a mistake in merging #5331 into the `2.7` branch as the bridge is also [compatible with the 2.3 version of the HttpFoundation component](https://github.com/symfony/psr-http-message-bridge/blob/master/composer.json#L21).

Commits
-------

fe9759a [PSR-7] Fix Diactoros link
c6f10c9 [#5331] Tiny typo
6df293c [PSR-7] Bridge documentation

Author: xabbuh

cookbook/index.rst

@@ -27,6 +27,7 @@ The Cookbook
     serializer
     service_container/index
     session/index
+    psr7
     symfony1
     templating/index
     testing/index

cookbook/map.rst.inc

@@ -193,6 +193,10 @@
   * (configuration) :doc:`/cookbook/configuration/mongodb_session_storage`
   * :doc:`/cookbook/session/avoid_session_start`
 
+* **PSR-7**
+
+  * :doc:`/cookbook/psr7`
+
 * **symfony1**
 
   * :doc:`/cookbook/symfony1`

cookbook/psr7.rst

@@ -0,0 +1,89 @@
+.. index::
+   single: PSR-7
+
+The PSR-7 Bridge
+================
+
+    The PSR-7 bridge converts :doc:`HttpFoundation </components/http_foundation/index>`
+    objects from and to objects implementing HTTP message interfaces defined
+    by the `PSR-7`_.
+
+Installation
+------------
+
+You can install the component in 2 different ways:
+
+* :doc:`Install it via Composer </components/using_components>` (``symfony/psr-http-message-bridge`` on `Packagist`_);
+* Use the official Git repository (https://github.com/symfony/psr-http-message-bridge).
+
+The bridge also needs a PSR-7 implementation to allow converting HttpFoundation
+objects to PSR-7 objects. It provides native support for `Zend Diactoros`_.
+Use Composer (``zendframework/zend-diactoros`` on `Packagist`_) or refer to
+the project documentation to install it.
+
+Usage
+-----
+
+Converting from HttpFoundation Objects to PSR-7
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The bridge provides an interface of a factory called
+:class:`Symfony\\Bridge\\PsrHttpMessage\\HttpMessageFactoryInterface`
+that builds objects implementing PSR-7 interfaces from HttpFoundation objects.
+It also provide a default implementation using Zend Diactoros internally.
+
+The following code snippet explain how to convert a :class:`Symfony\\Component\\HttpFoundation\\Request`
+to a Zend Diactoros :class:`Zend\\Diactoros\\ServerRequest` implementing the
+:class:`Psr\\Http\\Message\\ServerRequestInterface` interface::
+
+    use Symfony\Bridge\PsrHttpMessage\Factory\DiactorosFactory;
+    use Symfony\Component\HttpFoundation\Request;
+
+    $symfonyRequest = new Request(array(), array(), array(), array(), array(), array('HTTP_HOST' => 'dunglas.fr'), 'Content');
+    // The HTTP_HOST server key must be set to avoid an unexpected error
+
+    $psr7Factory = new DiactorosFactory();
+    $psrRequest = $psr7Factory->createRequest($symfonyRequest);
+
+And now from a :class:`Symfony\\Component\\HttpFoundation\\Response` to a Zend
+Diactoros :class:`Zend\\Diactoros\\Response` implementing the :class:`Psr\\Http\\Message\\ResponseInterface`
+interface::
+
+    use Symfony\Bridge\PsrHttpMessage\Factory\DiactorosFactory;
+    use Symfony\Component\HttpFoundation\Response;
+
+    $symfonyResponse = new Response('Content');
+
+    $psr7Factory = new DiactorosFactory();
+    $psrResponse = $psr7Factory->createResponse($symfonyResponse);
+
+Converting Objects implementing PSR-7 Interfaces to HttpFoundation
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+On the other hand, the bridge provide a factory interface called
+:class:`Symfony\\Bridge\\PsrHttpMessage\\HttpFoundationFactoryInterface`
+that builds HttpFoundation objects from objects implementing PSR-7 interfaces.
+
+The next snippet explain how to convert an object implementing the :class:`Psr\\Http\\Message\\ServerRequestInterface`
+interface to a :class:`Symfony\\Component\\HttpFoundation\\Request` instance::
+
+    use Symfony\Bridge\PsrHttpMessage\Factory\HttpFoundationFactory;
+
+    // $psrRequest is an instance of Psr\Http\Message\ServerRequestInterface
+
+    $httpFoundationFactory = new HttpFoundationFactory();
+    $symfonyRequest = $httpFoundationFactory->createRequest($psrRequest);
+
+From an object implementing the :class:`Psr\\Http\\Message\\ResponseInterface`
+to a :class:`Symfony\\Component\\HttpFoundation\\Response` instance::
+
+    use Symfony\Bridge\PsrHttpMessage\Factory\HttpFoundationFactory;
+
+    // $psrResponse is an instance of Psr\Http\Message\ResponseInterface
+
+    $httpFoundationFactory = new HttpFoundationFactory();
+    $symfonyResponse = $httpFoundationFactory->createResponse($psrResponse);
+
+.. _`PSR-7`: http://www.php-fig.org/psr/psr-7/
+.. _Packagist: https://packagist.org/packages/symfony/psr-http-message-bridge
+.. _`Zend Diactoros`: https://github.com/zendframework/zend-diactoros