Add a .gitattributes file
Add a .gitattributes file

This PR adds a `.gitattributes` file to keep the archives GH creates of the repo clean of development related files.
People using Composer can still get these files in their setup if they really want to, by using `--prefer-source`.

Refs:
* [Reddit: I don't need your tests in my production](https://www.reddit.com/r/PHP/comments/2jzp6k/i_dont_need_your_tests_in_my_production)
* [Blog: I don't need your tests in my production](https://blog.madewithlove.be/post/gitattributes/)

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