text change in readme.txt

git-svn-id: https://plugins.svn.wordpress.org/woocommerce-bulk-discount/trunk@856526 b8457f37-d9ea-0310-8a92-e5e31aec5664
version warning message updated

git-svn-id: https://plugins.svn.wordpress.org/woocommerce-bulk-discount/trunk@859513 b8457f37-d9ea-0310-8a92-e5e31aec5664