Add .gitattributes

This plugin should have a .gitattributes file.
This file can be used to to used to influenace certain operations perforemd by git for certain paths.

More information:

- https://git-scm.com/docs/gitattributes

Reference:

- https://github.com/orca-services/php-datatrans-xml-api-client/blob/master/.gitattributes