Menu Customizer: readme updates given current project status and WordPress 4.0 Beta 1.

git-svn-id: https://plugins.svn.wordpress.org/menu-customizer/trunk@948423 b8457f37-d9ea-0310-8a92-e5e31aec5664
Menu Customizer: version bump.

git-svn-id: https://plugins.svn.wordpress.org/menu-customizer/trunk@948428 b8457f37-d9ea-0310-8a92-e5e31aec5664