Adding some additional installation details

Hi guys!

This follows this conversation: https://groups.google.com/forum/#!topic/twig-users/B3NB8zU0Mg8

It doesn't actually say in the docs how to install the extension or that you need to add the extensions into Twig. That's especially confusing if you just somehow end up on this page: http://twig.sensiolabs.org/doc/extensions/index.html, which somewhat looks like core Twig functionality.

This compiled locally without any warnings and looked fine to me.

Thanks!