You can add a Welcome Message to your Help Center by going to Admin > Help Center > Help Center Design. Under Theme Options you will be able to input a Welcome Message title and Welcome Message content.
While editing, a live preview on the right that will update with your changes.
Make sure to press Save at the bottom of the screen to push any changes live on your Help Center.
For more information on editing your Help Center, check out our Guide: Help Center Preview and Publishing