Add publish date

Author: jaredhoberock

front_matter.html

@@ -1,6 +1,6 @@
 <cxx-titlepage stage="draft">
 <cxx-docnum>N3989</cxx-docnum>
-  <time pubdate="">2014-05-XX</time>
+  <time pubdate="">2014-05-23</time>
   <cxx-revises><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3960.pdf">N3960</a></cxx-revises>
   <cxx-editor>
     Jared Hoberock<br/>