[fix #19] Updated installation instructions

PR for issue #19 

Following the docs, everything suggest to put the bundle only on dev environment: add it to composer require-dev, register the bundle only when the kernel is in test/dev bundle. This is correct as long as you don't use block.html.twig. If you do, when going to production all error pages (4xx, 5xx) result in a ugly white-screen 500 page, since Twig fails to load the template.