minor #1428 Add Symfony Twig Brigde installation in asset doc (benjaminjonard)

This PR was merged into the 2.0.x-dev branch.

Discussion
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Add Symfony Twig Brigde installation in asset doc

Add Symfony Twig Brigde installation in asset doc, I have the following error otherwise : `Unknown "asset" function`
Made it clearer that twig-bridge is required.

Commits
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76bfd014 Add Symfony Twig Brigde installation in asset doc

doc/providers/asset.rst

@@ -43,6 +43,13 @@ Registering
 
         composer require symfony/asset
 
+    If you want to use assets in your Twig templates, you must also install the
+    Symfony Twig Bridge:
+
+    .. code-block:: bash
+
+        composer require symfony/twig-bridge
+
 Usage
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