Add gitattributes to reduce installed size of the project

[pieterocp]

This should reduce the size of this package when downloaded to vendor directories.

I don't think that any of these files are required for this package to be used, mostly helpful when developing.

(see https://php.watch/articles/composer-gitattributes for more info)

Finally implements - #157 (comment) also so people's security tools post-install won't moan.