TwigComponent: explain how to define default values

Author: alexislefebvre

src/TwigComponent/doc/index.rst

@@ -718,6 +718,29 @@ To tell the system that ``icon`` and ``type`` are props and not attributes, use
 
 .. _embedded-components:
 
+Default values
+~~~~~~~~~~~~~~
+
+The ``{% props %}`` tag can also be used to define default values of an anonymous component.
+
+.. code-block:: html+twig
+
+    {# templates/components/Image.html.twig #}
+    {% props size = 24 %}
+    
+    <img src="/image.jpg" width="{{ size }}" height="{{ size }}" alt="">
+
+Then you can call the component with or without defining this value:
+
+.. code-block:: html+twig
+
+    <twig:Image size="48"/>
+    <twig:Image/>
+    
+    {# renders as: #}
+    <img src="/image.jpg" width="48" height="48" alt="">
+    <img src="/image.jpg" width="24" height="24" alt="">
+
 Passing HTML to Components Via Blocks
 -------------------------------------