Updated readme.txt with better instructions

git-svn-id: https://plugins.svn.wordpress.org/shortcodes-to-show-or-hide-content/trunk@1000602 b8457f37-d9ea-0310-8a92-e5e31aec5664
Minor tweak to readme.txt

git-svn-id: https://plugins.svn.wordpress.org/shortcodes-to-show-or-hide-content/trunk@1000603 b8457f37-d9ea-0310-8a92-e5e31aec5664