add a "work in progress" note to README (#439)
add a "work in progress" note to README (#439)

This might help set user expectations.
angular/tsickle,alexeagle/tsickle,drjanitor/tsickle,ncfxy/tsickle,ncfxy/tsickle,alexeagle/tsickle,lucidsoftware/tsickle,ncfxy/tsickle,lucidsoftware/tsickle,lucidsoftware/tsickle,angular/tsickle,drjanitor/tsickle,drjanitor/tsickle,alexeagle/tsickle,drjanitor/tsickle