I would like to draw a Sydnone with chemfig. How do I get the -+ into the ring?

\documentclass{article}
\usepackage{tikz}
\usepackage{chemfig}
\begin{document}
\chemfig{N**5(-[,,1]\chembelow{N}{H}-[,,1]-[,3,1](=[,1,1]O)-[,3,]O-[,3])}
\end{document}
\documentclass{...}and ending with\end{document}. – DG' Apr 16 '15 at 11:56