I love me some cross-overs. And Archie is no stranger to them, having come face-to-face with both The Punisher and The Predator at different times in his career. So it's not much of a surprise that he's going to have a crossover with Supergirl, who herself is known to pal around with Flash every now-and-then. The fact that they're both CW shows also helps.
Anyway, here's confirmation from RIVERDALE showrunner Roberto Aguirre-Sacasa:
I'm actually of two minds about this. On the one hand, like I said I love crossovers (even the infamous CRITIC/SIMPSONS one), so that doesn't bother me, and also the SUPERGIRL/FLASH ones have been fun. However, I like how (relatively) grounded the RIVERDALE show is, and to all-of-sudden reveal that superheroes and supervillains are a thing? That might strain the stakes of a murder-mystery. But who knows how all this will play out. Hopefully it will be good.
Meanwhile, RIVERDALE season 2 will premiere on the CW October 11th.