Add missing credit to Mark Voorhies

docs/source/about/credits.txt

@@ -107,6 +107,8 b' And last but not least, all the kind IPython contributors who have contributed'
 new code, bug reports, fixes, comments and ideas. A brief list follows, please
 let us know if we have omitted your name by accident:
 
+* Mark Voorhies <mark.voorhies-AT-ucsf.edu> Printing support in Qt console.
+
 * Thomas Kluyver <takowl-AT-gmail.com> Port of IPython and its necessary ZeroMQ
   infrastructure to Python3.