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

Discussion
----------

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
-------

76bfd014 Add Symfony Twig Brigde installation in asset doc