Having something there would definitely be wise I think, given how many people are apparently just being sent links direct to a specific instance to sign up (and therefore skipping join-lemmy entirely). But it would need to be very short, to the point, and sum things up in a way non-techy folks can immediately understand. In my experience so far, a lot of people here struggle to explain the basic concept without writing a whole esseay about it, and nobody is gonna read a wall of text on the signup page.