updated readme.txt description

git-svn-id: https://plugins.svn.wordpress.org/facebook-ads/trunk@861979 b8457f37-d9ea-0310-8a92-e5e31aec5664
updated readme.txt

git-svn-id: https://plugins.svn.wordpress.org/facebook-ads/trunk@862717 b8457f37-d9ea-0310-8a92-e5e31aec5664
updated readme

git-svn-id: https://plugins.svn.wordpress.org/facebook-ads/trunk@864389 b8457f37-d9ea-0310-8a92-e5e31aec5664
updated readme headline

git-svn-id: https://plugins.svn.wordpress.org/facebook-ads/trunk@866308 b8457f37-d9ea-0310-8a92-e5e31aec5664