Added the release dates for the upcoming Symfony 3 versions
feature #4443 Added the release dates for the upcoming Symfony 3 versions (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
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Added the release dates for the upcoming Symfony 3 versions

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

Commits
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27abf4e Added the release dates for the upcoming Symfony 3 versions