bug #6347 Add a note about enabling DebugBundle to use VarDumper inside Symfony (martijn80, javiereguiluz)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes #6347).

Discussion
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Add a note about enabling DebugBundle to use VarDumper inside Symfony

This supersedes #6165 tweaking the proposed note contents.

Commits
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00bc64c Add a note about enabling DebugBundle to use VarDumper inside Symfony
2c79c08 Update introduction.rst

Author: weaverryan

components/var_dumper/introduction.rst

@@ -20,6 +20,11 @@ You can install the component in 2 different ways:
 * :doc:`Install it via Composer </components/using_components>` (``symfony/var-dumper`` on `Packagist`_);
 * Use the official Git repository (https://github.com/symfony/var-dumper).
 
+.. note::
+
+    If using it inside a Symfony application, make sure that the
+    DebugBundle is enabled in your ``app/AppKernel.php`` file.
+
 .. _components-var-dumper-dump:
 
 The dump() Function