update contributors

Author: Nathan Marz

README.markdown

@@ -52,6 +52,7 @@ You must not remove this notice, or any other, from this software.
 * Fabian Neumann ([@hellp](https://github.com/hellp))
 * Soren Macbeth ([@sorenmacbeth](https://github.com/sorenmacbeth))
 * Ashley  Brown ([@ashleywbrown](https://github.com/ashleywbrown))
+* Guanpeng Xu ([@herberteuler](https://github.com/herberteuler))
 
 ## Acknowledgements