INSTALLATION: Add a section on GHC (#744)
INSTALLATION: Add a section on GHC (#744)

Shortly described how GHC installation works with stack, and how to
check that GHC is installed correctly. Also added a "cautionary" note
about trying to run the Exercism exercises with a non-stack GHC (i.e.
might work, but stack is easiest).

Closes https://github.com/exercism/haskell/issues/608

exercism/xhaskell