Add zenzium to user showcase

Author: knolleary

index.html

@@ -121,6 +121,7 @@ <h4>In the cloud</h4>
         <div class="content">
             <h3>Who's using Node-RED?</h3>
             <ul id="user-list">
+                <li><a href="https://zenzium.com"><img src="users/zenzium.jpg"/></a></li>
                 <li><a href="https://www.vigilent.com"><img src="users/vigilent.png"/></a></li>
                 <li><a href="https://entelligently.com"><img src="users/entelligently.png"/></a></li>
                 <li><a href="https://st-one.io/"><img src="users/st-one.png"/></a></li>