add tip about indentation for emacs users

Signed-off-by: Arnaud Bailly <arnaud.oqube@gmail.com>

doc/coding_style.mdwn

@@ -30,6 +30,17 @@ each indented with a tab.
 		foo = ...
 		bar = ...
 
+Note for emacs users: You can put the following snippet into a file called 
+`.dir-locals.el` at root of propellor's source tree to ensure these indentation
+rules are enforced:
+
+	((nil . ((indent-tabs-mode . t)
+	         (tab-width . 4)
+	         (fill-column . 80)))
+	 ;; Warn about spaces used for indentation:
+	 (haskell-mode . ((eval . (highlight-regexp "^ *")))))
+
+
 Where clauses for instance definitions and modules tend to appear at the end
 of a line, rather than on a separate line.