add .gitattributes file

[Bernhard Döbler]

I see the slimphp/slim repository has a .gitattributes file that excludes tests from the composer download when --prefer-dist is provided. Can you maybe add this file to this repository, too? And also maybe to /Twig-View and /Slim-Csrf? Thank you.

[Rob Allen]

We'd love PRs for these :)