Update Yaml docs to list the features not supported

[javiereguiluz]

The Yaml component purpose is to parse the config files in Symfony apps. We have added some additional features to make it more useful, but we'll never make it spec-compliant.

Let's improve the Yaml docs adding this note about the purpose of the component and let's list the main unsupported features (such as symfony/symfony#12095).

[javiereguiluz]

@xabbuh and @GuilhemN you are our biggest YAML experts. Maybe you can quickly scan the TOC of the YAML 1.2 spec here http://www.yaml.org/spec/1.2/spec.html and tell me the most important features not supported by the YAML component? Then I'll make the necessary changes in the docs. Thanks!

[GuilhemN]

• Multi documents aren't supported (--- and ... markers in a YAML file).
• Complex mapping keys (keys beginning with ?) and complex values (beginning with ? too)
• Non-scalars mapping keys and soon non-string/non-int keys (in 4.0)
• Tagged values as keys
• !!set, !!omap, !!pairs, !!set, !!seq, !!bool, !!int, !!merge, !!null, !!timestamp, !!value, !!yaml (see http://www.yaml.org/type/)
• Global tags (TAG directive %TAG ! tag:example.com,2000:app/ and global references to a tag !<tag:example.com,2000:app/foo>)
• Not having a key in a mapping ({foo, bar} should be parsed as {foo: ~, bar: ~})