add custom content to appearance/widgets screen

found a hook here: http://hookr.io/actions/widgets_admin_page/

// define the widgets_admin_page callback 
function action_widgets_admin_page(  ) { 
    // make action magic happen here... 
}; 

// add the action 
add_action( 'widgets_admin_page', 'action_widgets_admin_page', 10, 0 );