Document how to create an extension

Author: greg0ire

docs/extension/index.rst

@@ -4,6 +4,28 @@
 Extending the phpdocumentor/guides
 ==================================
 
+``phpdocumentor/guides`` relies on `Symfony Dependency Injection
+Container
+<https://symfony.com/doc/current/components/dependency_injection.html#setting-up-the-container-with-configuration-files>`__
+extensions. This means that to extend the guides, you need to define
+such an extension, after what it becomes possible to make the guides CLI
+aware of it by creating a ``guides.xml`` file in the directory from
+which you invoke the CLI.
+
+It should look like this::
+
+.. code-block:: xml
+
+    <?xml version="1.0" encoding="UTF-8" ?>
+    <guides>
+        <extension class="YourName\YourExtension\DependencyInjection\YourExtension"/>
+    </guides>
+
+Internally, the guides CLI defines and uses default extensions.
+Once you have that set up, you can create PHP classes, define services
+from it, and tag them so that they are recognized and usable by the
+guides CLI.
+
 Some ways to extend the guides:
 
 .. toctree::