minor #3447 Adding link to RFC (ThomasLandauer)
minor #3447 Adding link to RFC (ThomasLandauer)

This PR was squashed before being merged into the 2.x branch.

Discussion
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Adding link to RFC

Reason for rewording the `twig/extra-bundle` part: It was not clear what "use" means here:

> Then, use the twig/extra-bundle on Symfony projects

Commits
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56fd57b3 Adding link to RFC

twigphp/Twig,twigphp/Twig,twigphp/Twig