Add property tree to root index file.

[SVN r56209]

index.html

@@ -55,8 +55,8 @@
 
   <h3>Changes in this release</h3>
 
-  <p>This release includes <span style="background-color: #FFFF00">???</span> 
-  new libraries as well as
+  <p>This release includes 1 new library
+  (<a href="libs/property_tree/index.html">property tree</a>) as well as
   updates to many existing libraries. See
   <a href="http://www.boost.org/users/history/version_1_41_0">Release
   History</a> for more information.</p>