This script will add a Button to the header. Adding Script to JIRA is a bit trciky, simplest solution is to place it in the Banner:
<script> AJS.toInit(function(){ AJS.$('.aui-header-primary .aui-nav').prepend('<li><a href="http://www.atlassian.com" class=" aui-nav-imagelink">Atlassian</a></li>'); }); </script>
PS: Due to XXS it will not show when You are in the admin section.