Added a note about the lacking features of Yaml Component
minor #4208 Added a note about the lacking features of Yaml Component (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
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Added a note about the lacking features of Yaml Component

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.3+
| Fixed tickets | -

In `symfony/symfony` repository there are a lot of issues reporting errors of the Yaml Component. These aren't errors but lacking features of the Yaml Component.

This PR tries to clearly state that Yaml Component lacks lots of features on purpose and provides a simple list of the most notable missing features.

Commits
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fe45194 Added a note about the lacking features of Yaml Component