some tweaks to bring README.md more up to date
some tweaks to bring README.md more up to date

Remove IRC link since we no longer idle there, but link the mailing list as
a way to stay in touch.

Also note that unstable package is required for Racket >= 6.3.

jackfirth/herbie,jackfirth/herbie,jackfirth/herbie,jackfirth/herbie,jackfirth/herbie,jackfirth/herbie