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 | -
This needs to be indented by four spaces and I think it should be reworded a bit like this:

> When creating the project, the Symfony Installer will check if your system meets the requirements that have to be fulfilled to run your new Symfony application. You can later on perform the requirements check again at will using the Symfony console application:

What do you think?