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 syl20bnr#1718
- add FAQ section
  - add note about setting up =stack-ghci= as =haskell-process= (as
  noted in syl20bnr#2142)
  - add note about fixing problems with =ghc-mod= and =stack=
  - add note about fixing stuck REPL (following syl20bnr#3271)

layers/+lang/haskell/README.org

@@ -21,6 +21,10 @@
    - [[#repl][REPL]]
    - [[#cabal-commands][Cabal commands]]
    - [[#cabal-files][Cabal files]]
+ - [[#faq][FAQ]]
+   - [[#repl-doesnt-work][REPL doesn't work.]]
+   - [[#i-am-using-stack-and-ghc-mod-but-ghc-mod-doesnt-work][I am using =stack= and =ghc-mod=, but =ghc-mod= doesn't work.]]
+   - [[#repl-is-stuck][REPL is stuck.]]
 
 * Description
 This layer adds support for the [[https://www.haskell.org/][Haskell]] language.
@@ -68,7 +72,7 @@ using =stack= then it should be =~/.local/bin=. So if this path is not in
 =$PATH=, just add it and Spacemacs will automatically pick up shell =$PATH=.
 
 _Alternatively_, you can add it to the Emacs =exec-path= variable in the
-=dotspacemacs/user-init= function of your =.spacemacs= file:
+=dotspacemacs/user-config= function of your =.spacemacs= file:
 
 #+BEGIN_SRC emacs-lisp
   (add-to-list 'exec-path "~/.cabal/bin/")
@@ -80,9 +84,6 @@ Or for =stack= users:
   (add-to-list 'exec-path "~/.local/bin/")
 #+END_SRC
 
-*Note:* it is important to add the path in the =dotspacemacs/user-init=
-function, so that the path is added before any layers is loaded.
-
 ** OS X
 Note that =emacs.app= for OS X does not pick up =$PATH= from =~/.bashrc= or
 =~/.zshrc= when launched from outside a terminal.
@@ -265,3 +266,32 @@ This commands are available in a cabal file.
 | ~SPC m N~   | go to next section                          |
 | ~SPC m P~   | go to previous section                      |
 | ~SPC m f~   | find or create source-file under the cursor |
+
+* FAQ
+
+** REPL doesn't work.
+
+Usually =haskell-mode= is great at figuring out which interactive process to
+bring up. But if you are experiencing problems with it you can help
+=haskell-mode= by setting =haskell-process-type= as in following code:
+
+#+BEGIN_SRC emacs-lisp
+  (setq-default dotspacemacs-configuration-layers
+    '((haskell :variables haskell-process-type 'stack-ghci)))
+#+END_SRC
+
+Available options are:
+
+- ghci
+- cabal-repl
+- cabal-dev
+- cabal-ghci
+- stack-ghci
+
+** I am using =stack= and =ghc-mod=, but =ghc-mod= doesn't work.
+
+Make sure that =dist= directory doesn't exist in your project root. So if it exists, just remove it and try again.
+
+** REPL is stuck.
+
+Make sure that when you are typing anything in REPL there is a space between what you type and =λ=. This problem usually happens when you switch from normal mode to edit mode.