add symfony/yaml suggestion to composer.json

The config:debug and yaml:lint commands use classes from the Yaml
component. Therefore, the installation of these packages should be
suggested to the user.

src/Symfony/Bundle/FrameworkBundle/composer.json

@@ -43,6 +43,7 @@
         "symfony/finder": "For using the translation loader and cache warmer",
         "symfony/form": "For using forms",
         "symfony/validator": "For using validation",
+        "symfony/yaml": "For using the config:debug and yaml:lint commands",
         "doctrine/cache": "For using alternative cache drivers"
     },
     "autoload": {