Add note about deprecation in the README

[Nek-]

This project is no longer maintained and contains a lot of issues (besides the work here is really good). I think recommending the improved version is just avoiding people a significant waste of time.

I can improve the sentence if you want. Could be something like This project is no longer maintained and has been forked in another repository...

[DrSnuggles]

Thanks for pointing to: https://github.com/ericblade/quagga2

[Florian-Schoenherr]

Why is nothing pointing to the new lib? This baffles me.