Add note on production use requiring Linux with systemd

index.html

@@ -118,6 +118,8 @@
 
       <pre><code>site <span class="token keyword">enable</span></code></pre>
 
+      <p><strong>The production server feature requires Linux with <a href='https://www.freedesktop.org/wiki/Software/systemd/'>systemd</a>.</strong> We use Site.js in production at <a href='https://small-tech.org'>Small Technology Foundation</a> on Ubuntu 18.04 LTS.</p>
+
       <p>Once your server is running, it will survive crashes and server restarts and you can use the following commands:</p>
 
       <ul>