Fix #708: add reference to Pandoc.

doc/intro.rst

@@ -35,6 +35,9 @@ to reStructuredText/Sphinx from other documentation systems.
 * Marcin Wojdyr has written a script to convert Docbook to reST with Sphinx
   markup; it is at `Google Code <http://code.google.com/p/db2rst/>`_.
 
+* To convert different markups, `Pandoc <http://johnmacfarlane.net/pandoc/>`_ is
+  a very helpful tool.
+
 
 Use with other systems
 ----------------------