By Uriel Sanchez ACAPULCO, Mexico (Reuters) – The severed heads of six men were discovered on Thursday atop a car in a town in southwestern Mexico with a sign warning others they could face the same fate, authorities said, a grisly reminder of the gang violence plaguing the country. A statement from the attorney general’s office of the state of Guerrero said the remains were found on a Volkswagen abandoned on a busy boulevard in the town of Chilapa de Alvarez. “In Chilapa selling crystal, kidnapping, extortion and stealing are strictly prohibited. This will happen to anyone who messes around,”…