added screenshots and readme additions

git-svn-id: https://plugins.svn.wordpress.org/conekta-woocommerce/trunk@989244 b8457f37-d9ea-0310-8a92-e5e31aec5664
readme additions

git-svn-id: https://plugins.svn.wordpress.org/conekta-woocommerce/trunk@989245 b8457f37-d9ea-0310-8a92-e5e31aec5664