Yaml format requires non-dev symfony/yaml in composer.json

[koenhoeymans]

It seems that composer.json needs symfony/yaml as dependency for users who wish to use the yaml format, not just as a dev dependency (which are not installed by default anymore).

[jrondorf]

It's already mentioned as suggested dependency in composer.json

[koenhoeymans]

Maybe it would be useful to mention it in the README in the section? I don't remember seeing the 'suggested' part after installing through composer and then the code examples failed with yaml so I had to figure out what was wrong myself. Not that it was much work but were it mentioned in the usage section I wouldn't have had a problem.