feature #3972 Added PSR-4 to Class Loaders list (dosten)

This PR was merged into the 2.5 branch.

Discussion
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Added PSR-4 to Class Loaders list

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | 2.5+
| Fixed tickets | none

Commits
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d1c95ce Added PSR-4 ClassLoader to list

components/class_loader/introduction.rst

@@ -12,11 +12,14 @@ Usage
 
 Whenever you reference a class that has not been required or included yet,
 PHP uses the `autoloading mechanism`_ to delegate the loading of a file defining
-the class. Symfony2 provides two autoloaders, which are able to load your classes:
+the class. Symfony2 provides three autoloaders, which are able to load your classes:
 
 * :doc:`/components/class_loader/class_loader`: loads classes that follow
   the `PSR-0` class naming standard;
 
+* :doc:`/components/class_loader/psr4_class_loader`: loads classes that follow
+  the `PSR-4` class naming standard;
+
 * :doc:`/components/class_loader/map_class_loader`: loads classes using
   a static map from class name to file path.