As Guriyot Singh wrote,
the "banner" has not been very detailed..... however the question has got a +1 !
So I try to guess
![smiley smiley](http://www.question2answer.org/qa/qa-plugin/wysiwyg-editor/plugins/smiley/images/regular_smile.gif)
It's this one !
![enlightened enlightened](http://www.question2answer.org/qa/qa-plugin/wysiwyg-editor/plugins/smiley/images/lightbulb.gif)
In case it is , it's not a plugin. It belongs to the core of q2a, and you can enable it in the Admin panel :
Admin ---> Users ---> Notice at top for first time visitors
or
Admin ---> Users ---> Notice at top for new registered users
There you can insert your custom message, using HTML too, for the 2 different types of notice.
I hope I've guessed !
![smiley smiley](http://www.question2answer.org/qa/qa-plugin/wysiwyg-editor/plugins/smiley/images/regular_smile.gif)