Fixed broken links in README
Fixed broken links in README

As the issue tracker of hackage seems to have moved to github without migrating the old issues, I have linked to the corresponding mail in the mail archive.
dmalikov/cabal2nix,spencerjanssen/cabal2nix,Fuuzetsu/cabal2nix,gridaphobe/cabal2nix,jb55/cabal2nix,psibi/cabal2nix,bennofs/cabal2nix