Android Auto works with Android 5.x devices. But if you're really not sure whether your phone is compatible, there's a tool at android.com/auto/check that'll tell you if you're good to go.
Unfortunately, it's broken. It's reporting back that the Nexus 6, Moto X 2014, HTC One M9 and HTC One M8 are not compatible with Android Auto. But we've used every single one of those phones with Android Auto. (And we're testing more as time permits.) So for now, don't rely on that check page. We'll update when it's fixed.