added a paragraph about experimental features

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.3+
| Fixed tickets | n/a

As Symfony 2.3 will freeze most of its API, we need a way to explicitly mark things as being experimental.

The goal of experimental features is to be able to gather feedback from our users as early as possible, by keeping the possibility to change things based on this feedback.

The first feature that will be marked as experimental in 2.3 is the simplification of the security customization (see symfony/symfony#6069 -- and Seldaek/symfony#3).