new quickbook docs for optional

[SVN r37809]

doc/dependencies.qbk

@@ -0,0 +1,17 @@
+[/
+    Boost.Optional
+
+    Copyright (c) 2003-2007 Fernando Luis Cacciola Carballal
+
+    Distributed under the Boost Software License, Version 1.0.
+    (See accompanying file LICENSE_1_0.txt or copy at
+    http://www.boost.org/LICENSE_1_0.txt)
+]
+
+
+[section Dependencies and Portability]
+
+The implementation uses `type_traits/alignment_of.hpp` and
+`type_traits/type_with_alignment.hpp`
+
+[endsect]