Added code example of how to use message factories (#130)

Added code example of how to use message factories

Author: Nyholm

message/message-factory.rst

@@ -29,3 +29,27 @@ This package provides interfaces for PSR-7 factories including:
 - ``StreamFactory``
 - ``UploadedFileFactory`` - WIP (PRs welcome)
 - ``UriFactory``
+
+Implementation for the interfaces above for `Diactoros`_ and `Guzzle PSR-7`_ can be found in ``php-http/message``.
+
+.. code:: php
+
+    // Create a PSR-7 request
+    $factory = new Http\Message\MessageFactory\DiactorosMessageFactory();
+    $request = $factory->createRequest('GET', 'http://example.com');
+
+    // Create a PSR-7 stream
+    $factory = new Http\Message\StreamFactory\DiactorosStreamFactory();
+    $stream = $factory->createStream('stream content');
+
+You could also use :doc:`/discovery` to find an installed factory automatically.
+
+.. code:: php
+
+    // Create a PSR-7 request
+    $factory = MessageFactoryDiscovery::find();
+    $request = $factory->createRequest('GET', 'http://example.com');
+
+
+.. _Diactoros: https://github.com/zendframework/zend-diactoros
+.. _Guzzle PSR-7: https://github.com/guzzle/psr7