Add .gitattributes

From [GitAttributes for PHP Composer Projects](https://php.watch/articles/composer-gitattributes)

Author: Ayesh

.gitattributes

@@ -0,0 +1,13 @@
+# https://php.watch/articles/composer-gitattributes
+
+# Exclude build/test files from archive
+/.github          export-ignore
+/tests            export-ignore
+/.editorconfig    export-ignore
+/.gitattributes   export-ignore
+/.gitignore       export-ignore
+/phpunit.xml.dist export-ignore
+
+# Configure diff output for .php and .phar files.
+*.php diff=php
+*.phar -diff