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 the choice up to the user. To have indentation an according indentation mode needs to be enabled. Otherwise, attempting to indent will print an error describing this.
+
+Minimal configuration may look something like:
+
+```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."