Update .gitattributes to ignore additional files

[sasezaki]

• With this PR, The archive targets other than src/ will be as follows:

gh api repos/sasezaki/chronos/tarball/patch-1 | tar -tzf - | cut -d/ -f2- | sed '/^$/d' | grep -v src/
LICENSE
README.md
composer.json
docs/
docs/config/
docs/config/__init__.py
docs/config/all.py
docs/en/
docs/en/3-x-migration-guide.rst
docs/en/conf.py
docs/en/contents.rst
docs/en/index.rst
docs/fr/
docs/fr/conf.py
docs/fr/contents.rst
docs/fr/index.rst
docs/ja/
docs/ja/conf.py
docs/ja/contents.rst
docs/ja/index.rst
docs/pt/
docs/pt/conf.py
docs/pt/contents.rst
docs/pt/index.rst

• IMO I think docs/ is unnecessary. If there is a demand, I will also export-ignore docs.

[ADmad]

docs can also be ignored from the archive.

[sasezaki]

docs can also be ignored from the archive.

just added

gh api repos/sasezaki/chronos/tarball/patch-1 | tar -tzf - | cut -d/ -f2- | sed '/^$/d' | grep -v src/
LICENSE
README.md
composer.json