Add Antonio Vargas Gonzalez to CONTRUIBUTORS.
# Please keep the list sorted.
Anthony Desnos <adesnos@google.com>
+Antonio Vargas Gonzalez <winden@google.com>
Christian Blichmann <cblichmann@google.com>
Hilko Bengen <bengen@hilluzination.de>
Joachim Metz <joachim.metz@gmail.com>
