doc updates

Author: kbond

src/Icons/README.md

@@ -37,6 +37,10 @@ from many different sets. This package provides a way to include any icon found
 2. Paste the snippet into your twig template and the icon will be automatically fetched (and cached).
 3. That's it!
 
+This works by using [Iconify's](https://iconify.design) API (to which [ux.symfony.com/icons](https://ux.symfony.com/icons)
+is a frontend for) to fetch the icon and render it in place. This icon is then cached for future requests
+for the same icon.
+
 > [!NOTE]
 > [Local SVG Icons](#local-svg-icons) of the same name will have precedence over _on-demand_ icons.