Added feature freeze dates for Symfony versions
minor #4259 Added feature freeze dates for Symfony versions (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
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Added feature freeze dates for Symfony versions

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

As requested by @webmozart [via Twitter](https://twitter.com/webmozart/status/514365470922014720).

Commits
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d157a14 Added feature freeze dates for Symfony versions