Add onion layer diagram

Author: igorw

source/css/style.css

@@ -77,6 +77,10 @@ pre code.php {
     }
 }
 
+figure {
+    text-align: center;
+}
+
 .authors {
     text-align: center;
 }

source/img/onion.png

@@ -52,12 +52,12 @@ <h2>What is Stack?</h2>
     <p>
         Stack is a convention for composing HttpKernelInterface middlewares.
     </p>
-
-    <!--
     <figure>
-        <img src="{{ site.url }}/img/middleware.png" alt="">
+        <img src="{{ site.url }}/img/onion.png" alt="Kernel Onion">
     </figure>
-    -->
+    <p>
+        By wrapping your application in decorators you can add new behaviour from the outside.
+    </p>
 </div>
 
 <hr>