Keep unnecessary artifacts out of releases

Author: Dominik Zogg

.gitattributes

@@ -0,0 +1,15 @@
+# This file was generated by the lean package validator (http://git.io/lean-package-validator).
+
+* text=auto eol=lf
+
+.gitattributes           export-ignore
+.github/                 export-ignore
+.gitignore               export-ignore
+.php-cs-fixer.php        export-ignore
+.php_cs                  export-ignore
+infection.json           export-ignore
+LICENSE                  export-ignore
+phpstan.neon             export-ignore
+phpunit.xml              export-ignore
+sonar-project.properties export-ignore
+tests/                   export-ignore