bug symfony#8804 [Templating] Add the asset package requirement (michaelperrin)

This PR was merged into the 4.0 branch.

Discussion
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[Templating] Add the asset package requirement

New comers will get an errors if they use the `asset()` Twig function without installing the `asset` package.

This PR makes sure they won't get an "WTF" reaction to the error they would get ;)

Commits
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47494ba Templating docs: add the asset package requirement

Author: weaverryan

templating.rst

@@ -738,7 +738,15 @@ Linking to Assets
 
 Templates also commonly refer to images, JavaScript, stylesheets and other
 assets. Of course you could hard-code the path to these assets (e.g. ``/images/logo.png``),
-but Symfony provides a more dynamic option via the ``asset()`` Twig function:
+but Symfony provides a more dynamic option via the ``asset()`` Twig function.
+
+To use this function, install the *asset* package:
+
+.. code-block:: terminal
+
+    $ composer require asset
+
+You can now use the ``asset()`` function:
 
 .. configuration-block::