Extend .gitattributes file

[swissspidy]

There's no need for CHANGELOG.md, CONTRIBUTING.md and phpcs.xml to be part of the package when installing the library via Composer.

[oscarotero]

I disagree.
CHANGELOG.md should be provided to any developer and consumer of any sofware. see this
CONTRIBUTING.md is not so important but is nice to have.
phpcs.xml can be ignored.

[swissspidy]

@oscarotero Fair enough. I now only added phpcs.xml to the list.

[oscarotero]

Thanks!