Make Git exclude some files when generating an archive

GitHub generates two archives (`.zip` and `.tar.gz`) for every tag / release we create. This is useful as we can provide the links to those archives to users to download a specific version of our project.

However, these archives contain all files the project had when the tag / release was made. This includes files that aren't of any use to the users, and are most likely going to be deleted (e.g.: `CONTRIBUTING.md`).

This commit whitelists the files that aren't useful for users and informs Git (and thus, GitHub) not to include them when generating an archive.

Ref: http://git-scm.com/book/en/Customizing-Git-Git-Attributes#Exporting-Your-Repository

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The current whitelisted files are (I'll update this list if we decide to change something): 

* `CHANGELOG.md` 
* `CONTRIBUTING.md`
* `img/.gitignore` 
* `LICENSE.md` 
* `README.md` 

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Thoughts?