be more precise in describing the installation

The installation section now uses the same style as the installation
instructions in the different component chapters. Thus, the section
now also covers the installation via Composer.
add compatibility warning on new features

Adds warnings that newer features are not compatible with Symfony
versions prior to 2.4. It also adds a description of the bundle's
release cycle to the installation instructions for reference purposes.
minor #319 add warning to feature descriptions being incompatible with Symfony 2.3 and earlier (xabbuh)

This PR was merged into the 3.0.x-dev branch.

Discussion
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add warning to feature descriptions being incompatible with Symfony 2.3 and earlier

Since the documentation on symfony.com renders the same SensioFrameworkExtraBundle documentation for each Symfony version, there are descriptions visible for Symfony versions prior to 2.4 that cannot be used (SensioFrameworkExtraBundle 3.0 and newer require Symfony 2.4 or higher).

This was reported to the Symfony repository (see symfony/symfony#11599).

Commits
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bd196d7 add compatibility warning on new features
32c1a4d be more precise in describing the installation