Add .gitattributes file with export ignore rules

[ocean90]

When you use git archive all the test and stub files are also exported. The archive should usually only contain the production files since this is also the package installed by Composer by default, see https://getcomposer.org/doc/05-repositories.md#package.

Testing instructions:

1. Checkout the PR
2. git archive -o latest.zip HEAD
3. unzip latest.zip
4. cd unzip
5. tree .

Expected output:

❯ tree .
.
├── composer.json
├── includes
│   ├── adapter-interface.php
│   ├── memcache-adapter.php
│   ├── memcached-adapter.php
│   ├── stats.php
│   └── wp-object-cache.php
├── object-cache.php
└── readme.md

2 directories, 8 files