Add William King to contributors

CONTRIBUTORS.txt

@@ -176,6 +176,7 @@ Nathan Van Gheem, 2014/10/28
 Gino Ledesma, 2014/10/28
 Thomas French, 2014/11/10
 Michael Permana, 2014/11/6
+William King, 2014/11/21
 Bert Vanderbauwhede, 2014/12/18
 John Anderson, 2014/12/27
 Luke Burden, 2015/01/24