There are many modules to customize status messages on a Drupal website. Such messages could inform your users of scheduled maintenance downtime or other relevant notices you want any user to see when they visit your website. I suggest you consult this community comparison on drupal status message modules.
If these are too heavyweight for you there is a very simple solution. In your theme’s template.php file add the following line of code anywhere in YOURTHEME/template.php: Learn More