add banner and screenshots

git-svn-id: https://plugins.svn.wordpress.org/woocommerce-product-tabs/trunk@1019679 b8457f37-d9ea-0310-8a92-e5e31aec5664
update README.txt

git-svn-id: https://plugins.svn.wordpress.org/woocommerce-product-tabs/trunk@1019682 b8457f37-d9ea-0310-8a92-e5e31aec5664