Added Upgrade Notice to the readme with more details of the upgrade process.

git-svn-id: https://plugins.svn.wordpress.org/google-calendar-widget/trunk@1029187 b8457f37-d9ea-0310-8a92-e5e31aec5664
Updated the version number in ko-calendar.php to 1.4.0.

git-svn-id: https://plugins.svn.wordpress.org/google-calendar-widget/trunk@1029191 b8457f37-d9ea-0310-8a92-e5e31aec5664