Added screenshot to readme

git-svn-id: https://plugins.svn.wordpress.org/woocommerce-payment-gateway-ccbill/trunk@1020874 b8457f37-d9ea-0310-8a92-e5e31aec5664
Updated screenshot

git-svn-id: https://plugins.svn.wordpress.org/woocommerce-payment-gateway-ccbill/trunk@1020876 b8457f37-d9ea-0310-8a92-e5e31aec5664