Add John Watson to CONTRIBUTORS

CONTRIBUTORS.txt

@@ -106,3 +106,4 @@ Dimitrios Kouzis-Loukas, 2012/06/13
 Steven Skoczen, 2012/06/17
 Loren Abrams, 2012/06/19
 Eran Rundstein, 2012/06/24
+John Watson, 2012/06/27