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.
minor #11623 add symfony/yaml suggestion to composer.json (xabbuh)

This PR was merged into the 2.5 branch.

Discussion
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add symfony/yaml suggestion to composer.json

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

Commits
-------

dcce035 add symfony/yaml suggestion to composer.json