Add Reveal.js license to package.json

package.json

@@ -33,5 +33,11 @@
     "grunt-contrib-watch": "~0.2.0",
     "grunt-contrib-sass": "~0.2.2",
     "grunt": "~0.4.0"
-  }
+  },
+  "licenses": [
+    {
+      "type": "MIT",
+      "url": "https://github.com/hakimel/reveal.js/blob/master/LICENSE"
+    }
+  ]
 }