Added .gitattributes

[batumibiz]

If Сomposer uses the parameter "optimize-autoloader": true, there is a warning:

Generating optimized autoload files
Class Psalm\MockeryPlugin\Tests\AcceptanceTester located in vendor/psalm/plugin-mockery\tests_support\AcceptanceTester.php does not comply with psr-4 autoloading standard. Skipping.
Class Psalm\MockeryPlugin\Tests\Helper\Acceptance located in vendor/psalm/plugin-mockery\tests_support\Helper\Acceptance.php does not comply with psr-4 autoloading standard. Skipping.

Adding .gitattributes solves this problem.
The file code is taken from here: https://github.com/psalm/psalm-plugin-phpunit

[weirdan]

Thanks!