Remove .dir-locals.el and add doc/contributing.mdwn.

Add link CONTRIBUTING -> doc/contributing.mdwn, so that it's easy to
find (many files in doc/).

Add .dir-locals.el to .gitignore, now that it's no longer versioned.

The CONTRIBUTING file gives a reference to a page on the Emacs wiki
that shows how to set up a .dir-locals.el that sets up tabs for
indentation.  I updated the wiki page to include the
`(highlight-regexp "^ *")` part, which had been the hardest to
discover.

.dir-locals.el

@@ -1,22 +0,0 @@
-;; Configure emacs' treatment of tabs.
-;;
-;; See
-;; https://www.gnu.org/software/emacs/manual/html_node/emacs/Directory-Variables.html
-;; for a description of this file.
-;;
-;; The 'nil' below applies to all modes.
-((nil . ((indent-tabs-mode . t)
-        (tab-width . 2)))
- (haskell-mode . (
-        ;; Highlight leading space characters, to avoid indenting with
-        ;; spaces.
-        ;;
-        ;; Emacs will prompt you about this, saying it's unsafe, but
-        ;; you can permanently store an exception by pressing "!",
-        ;; which inserts
-        ;;
-        ;;   (safe-local-variable-values . (quote ((eval highlight-regexp "^ *"))))
-        ;;
-        ;; in your ~/.emacs ... except the exception doesn't work, and
-        ;; emacs still asks you on each file you open :P
-        (eval . (highlight-regexp "^ *")))))

.gitignore

@@ -14,4 +14,7 @@ html
 Utility/Touch.hs
 Utility/libdiskfree.o
 dist
+# Sandboxed builds
 cabal-dev
+# Project-local emacs configuration
+.dir-locals.el

CONTRIBUTING

@@ -0,0 +1 @@
+doc/contributing.mdwn

doc/contributing.mdwn

@@ -0,0 +1,7 @@
+## Style
+
+This project uses tabs for indentation and the code looks fine with
+any tab width. If you are using Emacs, and have it configured to use
+spaces for indentation, then you can add a ./.dir-locals.el to use
+tabs for files in this project. See
+http://www.emacswiki.org/emacs/DirectoryVariables.