Be explicit about indentation mode not being enabled

Currently when a new user tries indenting, that basically invokes help
for the mode hook, which isn't useful and looks more like a bug than
anything else.

Solve this by printing an explicit explanation that a user needs to
enable an indentation mode.

Author: Hi-Angel

README.md

@@ -68,10 +68,19 @@ and recommended installation method.
 Basic Configuration
 -------------------
 
-For setup instructions, please consult the new integrated purescript-mode
-[Info](https://www.gnu.org/software/texinfo/manual/info/info.html)
-manual which can be accessed after installation via
-`M-x info-display-manual [RET] purescript-mode`.
+PureScript mode provides multiple indentation engines, and leaves it up to the user to chose one. To have indentation you need to add a hook that enables according mode. Otherwise, attempting to indent will just print an error about indentation mode not being enabled.
+
+Minimal configuration may look something like this:
+
+```lisp
+(use-package purescript-mode
+  :defer t
+  :config
+  (defun myhook-purescript-mode ()
+    (turn-on-purescript-indentation)
+    (add-hook 'before-save-hook #'purescript-sort-imports nil t))
+  (add-hook 'purescript-mode-hook 'myhook-purescript-mode))
+```
 
 Support
 -------

purescript-mode.el

@@ -407,7 +407,10 @@ is asked to show extra info for the items matching QUERY.."
 has been selected.
 
 Brings up the documentation for purescript-mode-hook."
-  (describe-variable 'purescript-mode-hook))
+  (error
+   "To use indentation you need to turn-on one of the indentation modes. Please see `purescript-mode-hook' documentation for examples.
+
+Currently `purescript-indentation' is the most maintained mode."))
 
 (defun purescript-mode-format-imports ()
   "Format the imports by aligning and sorting them."