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.