docs: update extending collection section with code example

docs/pages/code/extending-collection-with-collectiondecorator.php

@@ -0,0 +1,21 @@
+<?php
+
+declare(strict_types=1);
+
+include __DIR__ . '/../../../vendor/autoload.php';
+
+use loophp\collection\CollectionDecorator;
+
+final class FoobarCollection extends CollectionDecorator
+{
+    public function toUpperCase(): FoobarCollection
+    {
+        return $this
+            ->map(
+                static fn (string $letter): string => strtoupper($letter)
+            );
+    }
+}
+
+$collection = FoobarCollection::fromIterable(range('a', 'e'))
+    ->toUpperCase();

docs/pages/usage.rst

@@ -102,6 +102,7 @@ If you want to **extend** the Collection, you have multiple options.
 
 1. You just create a ``callable`` and use :ref:`pages/api:pipe` method.
 2. You use the Composition design pattern and create your own library class.
+3. Use the `CollectionDecorator` abstract class available since version 7.0.0
 
 Every classes of this library are ``final`` and then it is impossible to use
 inheritance and use the original Collection class as parent of another one.
@@ -115,6 +116,13 @@ new operation ``foobar``.
 .. literalinclude:: code/extending-collection.php
   :language: php
 
+Since version 7.0.0, we ship a `CollectionDecorator` abstract class which let
+you create your own Collection class with all your customizations.
+
+.. literalinclude:: code/extending-collection-with-collectiondecorator.php
+  :language: php
+
+
 Manipulate strings
 ~~~~~~~~~~~~~~~~~~