[.dir-locals.el] Add shell script mode

.dir-locals.el

@@ -5,5 +5,6 @@
             (c-basic-offset . 4)))
  (makefile-mode . ((indent-tabs-mode . t)))
  (asm-mode . ((indent-tabs-mode . t)))
+ (shell-script-mode . ((indent-tabs-mode . t)))
  ((nil . ((truncate-lines . t)))
           (text-mode . ((eval . ((turn-on-auto-fill)))))))