minor #7383 Added a note about not using the ClassLoader component (javiereguiluz)

This PR was squashed before being merged into the 2.7 branch (closes #7383).

Discussion
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Added a note about not using the ClassLoader component

This fixes #5949.

The component will be deprecated soon, so I guess it's OK to start warning our users.

Commits
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e96cbf8 Added a note about not using the ClassLoader component

Author: xabbuh

components/class_loader.rst

@@ -7,6 +7,11 @@ The ClassLoader Component
     The ClassLoader component provides tools to autoload your classes and
     cache their locations for performance.
 
+.. caution::
+
+    The ClassLoader component was deprecated in Symfony 3.3 and it will be
+    removed in 4.0. Alternatively, use Composer's class loading mechanism.
+
 Usage
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