Added shortcode support

git-svn-id: https://plugins.svn.wordpress.org/elokenz-most-shared-articles-for-authors/trunk@956696 b8457f37-d9ea-0310-8a92-e5e31aec5664
Added details to readme.txt

git-svn-id: https://plugins.svn.wordpress.org/elokenz-most-shared-articles-for-authors/trunk@956698 b8457f37-d9ea-0310-8a92-e5e31aec5664