Updated readme.txt file with info about "WordPress SEO by Yoast" including the Sitelink Search Box markup in their plugin from version 1.6 on. If you use WordPress SEO by Yoast version 1.6 or newer, you don't need this plugin any more.

git-svn-id: https://plugins.svn.wordpress.org/sitelinks-search-box/trunk@988490 b8457f37-d9ea-0310-8a92-e5e31aec5664