update readme file

git-svn-id: https://plugins.svn.wordpress.org/ig-pagebuilder/trunk@908801 b8457f37-d9ea-0310-8a92-e5e31aec5664
update readme file and preview image

git-svn-id: https://plugins.svn.wordpress.org/ig-pagebuilder/trunk@908802 b8457f37-d9ea-0310-8a92-e5e31aec5664