First shot of a documentation of the new PSR-4 class loader.

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#10100)
| Applies to    | 2.5+
| Fixed tickets | #3655

Hello docs team,

this is a first draft of a documentation for the PSR-4 class loaded I've contributed. As this is me first contribution to the documentation, any assistance would be appreciated. :-)