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.
