Add myself to Contributors (pull request 1142)

CONTRIBUTORS.txt

@@ -124,3 +124,4 @@ Sundar Raman, 2012/09/24
 Henri Colas, 2012/11/16
 Thomas Grainger, 2012/11/29
 Marius Gedminas, 2012/11/29
+Christoph Krybus, 2013/01/07