Mention Pandoc

`ronn` is not packaged in too many distributions (I know of Arch and Debian Unstable), whereas `pandoc` is packaged more widely (Debian, Ubuntu, Fedora).

People who search for `markdown to man page` find this project, but then might not find a package in their distribution (or Mac OS X or Windows even). It would be nice to tell them that there is a tool for markdown to roff conversion, although it's HTML does not look as slick as ronn's.