minor #17827 [Config] Add suggest to composer.json for symfony/yaml (Calin Mihai Pristavu)

This PR was merged into the 2.7 branch.

Discussion
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[Config] Add suggest to composer.json for symfony/yaml

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #17826
| License       | MIT

Fixes #17826

Commits
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2200b0c [Config] Add suggest to composer.json for symfony/yaml

Author: fabpot

composer.json

@@ -19,6 +19,9 @@
         "php": ">=5.3.9",
         "symfony/filesystem": "~2.3"
     },
+    "suggest": {
+        "symfony/yaml": "To use the yaml reference dumper"
+    },
     "autoload": {
         "psr-4": { "Symfony\\Component\\Config\\": "" },
         "exclude-from-classmap": [