Take a look at custom dashboard widget usage :
https://codex.wordpress.org/Dashboard_Widgets_API
https://codex.wordpress.org/Function_Reference/wp_add_dashboard_widget
http://www.wpbeginner.com/wp-themes/how-to-add-custom-dashboard-widgets-in-wordpress/
http://wpengineer.com/307/add-wordpress-dashboard-widgets/
I have included the code you need copied from the reference link 3rd link which will allow you to add a custom dashboard widget.
Add this to your functions.php and customise as you wish.
add_action('wp_dashboard_setup', 'my_custom_dashboard_widgets');
function my_custom_dashboard_widgets() {
global $wp_meta_boxes;
wp_add_dashboard_widget('custom_help_widget', 'Theme Support', 'custom_dashboard_help');
}
function custom_dashboard_help() {
echo '<p>Welcome to Custom Blog Theme! Need help? Contact the developer <a href="https://wordpress.stackexchange.com/questions/283508/mailto:[email protected]">here</a>. For WordPress Tutorials visit: <a href="http://www.wpbeginner.com" target="_blank">WPBeginner</a></p>';
}