The first international rock festival to be held in the Soviet Union began Friday and continues through the weekend at the Tallinn Amphitheater in Tallinn, Estonia. The festival, titled "Glasnost Rock '88" is free of charge and will include appearances by several performers from the West, including Public Image Ltd., Big Country, Steve Hackett and Leather Nun, plus bands from Italy, Sweden and Finland. A dozen Soviet acts share the bill, including Dialog, Nautilus Pompilus, Kruiz, Avgust and Zebra.