Jonathan Bardo

Note: This plugin only works with WordPress 4.2 or higher
This plugin will enable the site administrator to easily create an admin message present on all pages. The admin has the option to let the user dismiss the message for the duration of his session.

Admin User Message

This does just what I want. Thanks for making it. :)
It's definitely better than https://wordpress.org/plugins/admin-screen-messages
You might want to also tag it as:
https://wordpress.org/plugins/tags/admin-message



Perfect

Add message to users of your site. Choose wheter they can dismiss it or not.

Nicolas Bouliane

Displays a custom message on the dashboard with full HTML/PHP/JavaScript support. Useful for administrator who have clients as users.
The plugin will display anything that’s in the message.php file in the plugin’s directory. Since the plugin uses a simple include, 
any kind of content is supported.

Dashboard Message

Displays a custom message on the dashboard with full HTML/PHP/JavaScript support. Useful for administrator who have clients as users.

Admin User Message vs Dashboard Message