Update empty state copy

ee/app/views/shared/empty_states/_epics.html.haml

@@ -5,6 +5,8 @@
   .col-xs-12.text-center
     .text-content
       %h4
-        Epics let you manage your portfolio of projects more efficiently and with less effort.
+        Epics let you manage your portfolio of projects more efficiently and with less effort
+      %p
+        Track groups of issues that share a theme, across projects and milestones
       %button.btn.btn-new{ type: 'button' }
         New epic