added newsletter header button

_includes/header.html

@@ -33,11 +33,9 @@
           aria-label="Star SimBricks on GitHub"></a>
         </div>
       </li>
-      <!--
       <li class="newsletter-button">
         <a href="{{ site.baseurl }}/newsletter/" title="Keep me updated">Keep me updated</a>
       </li>
-      -->
       <li class="slack-button">
         <a
           href="https://join.slack.com/t/simbricks/shared_invite/zt-16y96155y-xspnVcm18EUkbUHDcSVonA"