update haskell README file
update haskell README file

- update section about setting up PATH by updating value of
  =exec-path= (previously it was saying to update it in =user-init=
  function, but looks like you need to do it in =user-config=) -
  following #1718
- add FAQ section
  - add note about setting up =stack-ghci= as =haskell-process= (as
  noted in #2142)
  - add note about fixing problems with =ghc-mod= and =stack=
  - add note about fixing stuck REPL (following #3271)

tsonntag/dotemacs,tsonntag/dotemacs,tsonntag/dotemacs,tsonntag/dotemacs