Add notice about new Playground.

Author: BigBlueHat

playground/index.html

@@ -80,6 +80,9 @@
 
     <div class="container">
       <h2>JSON-LD Playground</h2>
+      <div class="alert alert-success">
+        <strong><a href="next/">Checkout our next Playground!</a></strong>
+      </div>
       <p>
         Play around with JSON-LD markup by typing out some JSON below and
         seeing what gets generated from it at the bottom of the page. Pick any