minor #3577 Update README.md (noniagriconomie)

This PR was merged into the 3.x branch.

Discussion
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Update README.md

Ref twigphp/cache-extra#2
friendly ping `@fabpot`

Commits
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9286dac Update README.md

extra/cache-extra/README.md

@@ -4,4 +4,6 @@ Cache Extension
 This package is a Twig extension that provides integration with the Symfony
 Cache component.
 
-It provides a single `cache` tag that allows to cache template fragments.
+It provides a single [`cache`][1] tag that allows to cache template fragments.
+
+[1]: https://twig.symfony.com/cache