guzzle custom client + doc

Author: Younès El Biache

README.md

@@ -147,9 +147,11 @@ The resolver configuration is defined under the "resolver" key and it has two op
 * config: The options passed to the class. If you use the default curl class, the config are the same than the [curl_setopt PHP function](http://php.net/manual/en/function.curl-setopt.php)
 
 ```php
+
+// CURL
 $config = [
     'resolver' => [
-        'class' => 'Embed\\RequestResolvers\\Curl' //The default resolver used
+        'class' => 'Embed\\RequestResolvers\\Curl' // The default resolver used
 
         'config' => [
             CURLOPT_MAXREDIRS => 20,
@@ -164,6 +166,18 @@ $config = [
         ]
     ]
 ];
+
+// Guzzle (5.x)
+$config = [
+    'resolver' => [
+        'class' => 'Embed\\RequestResolvers\\Guzzle', // Guzzle resolver used
+
+        'config' => [
+            // optional: if you need to use your custom Guzzle instance
+            'client' => $myGuzzleClient, 
+        ]
+    ]
+];
 ```
 
 ### Image info

src/RequestResolvers/Guzzle.php

@@ -29,7 +29,7 @@ class Guzzle implements RequestResolverInterface
      */
     public function __construct($url, array $config)
     {
-        $this->client  = new Client($config ?: []);
+        $this->client  = isset($config['client']) ? $config['client'] : new Client();
         $this->request = $this->client->createRequest('GET', $url);
     }