Create manifest.json

I added this file to take care of the error message I received when installing roots on windows "Warning: file_get_contents(C:\xampp\apps\wordpress\htdocs/wp-content/themes/roots-master/assets/manifest.json): failed to open stream: No such file or directory in C:\xampp\apps\wordpress\htdocs\wp-content\themes\roots-master\lib\scripts.php on line 30". It's just any empty json file and fixes the error I was encountering.