Add custom Guzzle client section to docs

Author: ddeboer

doc/proxy-clients.rst

@@ -104,7 +104,7 @@ path::
  ;
 
 You can specify HTTP headers as the second argument to ``purge()``.
-For instance:
+For instance::
 
  $client
  ->purge('/some/path', array('X-Foo' => 'bar')
@@ -114,6 +114,10 @@ For instance:
 Please note that purge will invalidate all variants, so you do not have to
 send any headers that you vary on, such as ``Accept``.
 
+The above allows you to pass headers that are different between purge requests.
+If you want to send headers for all purge requests, such as ``Authorization``,
+use a :ref:`custom Guzzle client <custom Guzzle client>` instead.
+
 Refresh
 ~~~~~~~
 
@@ -174,3 +178,23 @@ Varnish client::
 
 Make sure to add any headers that you want to ban on to your
 :doc:`Varnish configuration <varnish-configuration>`.
+
+.. _custom guzzle client:
+
+Custom Guzzle Client
+--------------------
+
+By default, the proxy clients instantiate a `Guzzle client`_ to communicate
+with the caching proxy. If you need to customize the requests, for example to
+send a basic authentication header, you can inject a custom Guzzle client::
+
+ use FOS\HttpCache\ProxyClient\Varnish;
+ use Guzzle\Http\Client;
+
+ $client = new Client();
+ $client->setDefaultOption('auth', array('username', 'password', 'Digest'));
+
+ $servers = array('10.0.0.1');
+ $varnish = new Varnish($servers, '/baseUrl', $client);
+
+.. _Guzzle client: http://guzzle3.readthedocs.org/