Update README.md

README.md

@@ -77,6 +77,43 @@ My dog has fleas.
 
 See tests for more usage samples.
 
+## Extending
+
+You could need to provide your own class for a type, for example to implement https://www.w3.org/wiki/SocialCG/ActivityPub/Authentication_Authorization.
+
+Imagine you want an Service having a ```publicKey``` property.
+
+``` php
+use \Patbator\ActivityStreams\Model\Service;
+
+
+class My_ActivityPub_Service extends Service 
+{
+  public function __construct() 
+  {
+    parent::__construct();
+    $this->_actor_attribs[] = 'publicKey';
+  }
+
+
+  public function type()
+  {
+    return 'Service';
+  }
+}
+```
+
+You can declare it to type factory:
+
+``` php
+use Patbator\ActivityStreams\Model\Base;
+use Patbator\ActivityStreams\Model\Factory;
+
+Base::setFactory((new Factory)
+                 ->mapTypeToClass('Service', 'My_ActivityPub_Service'));
+```
+
+After that, factory will create a new ```My_ActiviyPub_Service``` for any json object with ```"type": "Service"``` instead of a ```\\Patbator\\ActivityStreams\\Model\\Service```.
 
 ## Testing