added proposed under construction banner (#57)

Author: dvidsilva

public/_includes/_main-nav.jade

@@ -16,3 +16,7 @@ md-toolbar(class="main-nav background-regal l-pinned-top l-layer-5",scroll-y-off
       li.l-right
         a.main-nav-button.md-button.ng-cloak(ng-click="appCtrl.toggleSource($event)", href)
           span {{appCtrl.sourceVisible?'Hide English':'Show English'}}
+
+h2.under-construction
+  p Esta página esta en construcción, contribuye en 
+    a(href="https://github.com/angular/angular-es") github

public/resources/css/module/_main-nav.scss

@@ -157,4 +157,14 @@
       display: inline-block;
     }
   }
-}
+}
+.under-construction {
+    text-align: center;
+    background: #0152b3;
+    color: white;
+    margin: 0;
+    padding: 1em 0;
+    p {
+        margin: 0;
+    }
+}