Recommend `cabal` instead of `stack`

[byorgey]

See discussion at #1798 (comment) and #1624 (comment) .

[kostmo]

Filed commercialhaskell/stack#6569

[byorgey]

Independently, it also seems that recently I haven't been able to get emacs + HLS to work at all with stack, but it works great with cabal, so I have personally switched away from using stack as well.