Note about checking symfony requirements by installer

Since version 0.2.0 Installer will also check for symfony requirements - I think it would be good to mention about it in docs.

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | addition about https://github.com/symfony/symfony-installer/pull/27 
| Applies to    | >=2.3
| Fixed tickets | -
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