The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. I. Discovery of the Optical Counterpart Using the Dark Energy Camera - Soares-Santos M; Holz DE; Annis J; Chornock R; Herner K; Berger E; Brout D; Chen HY; Kessler R; Sako M; Allam S; Tucker DL; et al. (Astrophysical Journal, 2017)