Tweak the contributing guidelines

Author: bbatsov

CONTRIBUTING.md

@@ -38,15 +38,20 @@ and description in grammatically correct, complete sentences.
 
 ## Development setup
 
-1. Fork and clone the repository
-1. Install [Cask][7] as per the instructions there.
-1. With Cask in your path run `cask install` in the repository folder.
+1. Fork and clone the repository.
+1. Install [Cask][7].
+1. Run `cask install` in the repository folder.
 1. Run tests with `make test`.
 
+**Note:** macOS users should make sure that the `emacs` command resolves the version of Emacs they've installed
+manually (e.g. via `homebrew`), instead of the ancient Emacs 22 that comes bundled with macOS.
+See [this article][8] for more details.
+
 [1]: https://github.com/clojure-emacs/clojure-mode/issues
 [2]: http://gun.io/blog/how-to-github-fork-branch-and-pull-request
 [3]: http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html
 [4]: https://help.github.com/articles/using-pull-requests
 [5]: http://gitready.com/advanced/2009/02/10/squashing-commits-with-rebase.html
 [6]: https://github.com/clojure-emacs/clojure-mode/blob/master/CHANGELOG.md
 [7]: https://github.com/cask/cask
+[8]: https://emacsredux.com/blog/2015/05/09/emacs-on-os-x/