Make Git exclude files when generating an archive

GitHub generates two archives (a `.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