Updated documentation

git-svn-id: https://plugins.svn.wordpress.org/pdf-thumbnails/trunk@1028800 b8457f37-d9ea-0310-8a92-e5e31aec5664
Fixed spelling error in readme

git-svn-id: https://plugins.svn.wordpress.org/pdf-thumbnails/trunk@1028804 b8457f37-d9ea-0310-8a92-e5e31aec5664