Update README.md

Author: dunglas

README.md

@@ -15,6 +15,7 @@ A [Docker](https://www.docker.com/)-based installer and runtime for the [Symfony
 ## Features
 
 * Production, development and CI ready
+* [Installation of extra Docker Compose services](docs/extra-services.md) with Symfony Flex
 * Automatic HTTPS (in dev and in prod!)
 * HTTP/2, HTTP/3 and [Preload](https://symfony.com/doc/current/web_link.html) support
 * Built-in [Mercure](https://symfony.com/doc/current/mercure.html) hub