Create .gitattributes to exclude non-essential files

This reduces the amount of files downloaded/used when installing the package through composer without losing any functionality. The size is reduced from about 25.5MB to 0.25MB, saving lots of bandwidth and space when used.

Author: iquito

.gitattributes

@@ -0,0 +1,9 @@
+/.github export-ignore
+/docs export-ignore
+/tests export-ignore
+.gitattributes export-ignore
+.gitignore export-ignore
+.php-cs-fixer.dist.php export-ignore
+phpstan.neon.dist export-ignore
+phpunit.xml.dist export-ignore
+rector.php export-ignore