Let's add a redirect notice pointing at react-snap

[mcmonster]

geelen, congrats on building this project - reading through the async future ticket, it seems like you were working towards building something very cool.

After reading through #80 and the overall lack of commits lately, it seems that react-snap is the future moving forward. I landed here in search of static prerendering and didn't notice the aging state of the repo until I hit the react-16 wall.

I'd like to propose putting a notice at the top of the README to let those who come after know that react-snap might be the better route to go. I'm happy to send a PR for the notice.