They're already stuck in the minds of millions of readers. But now Zonker, Mike, Duke and the gang have found a new place to hang out--on stamps. Cartoonist Garry Trudeau and his associate George Corsillo have come up with "The 1990 Doonesbury Stamp Album" as a way to raise money for the Literacy Volunteers of New York City, Inc. The stamps also mark 20 years of "Doonesbury" antics. But don't try to send mail with them. Like the strip, they're not for real.