Merge pull request #419 from dret/patch-5

abstract needs a little love

Author: rhiaro

core/index.html

@@ -117,7 +117,9 @@
   <section id="abstract">
     <p>
       This specification details a model for representing potential and
-      completed activities using the JSON format.
+      completed activities using the JSON format. It is intended to be used
+      with vocabularies that detail the structure of activities, and define
+      specific types of activities.
     </p>
 
     <section id="authorsnote" class="informative">