Document support for macOS

This adds docs for b0c85eddf6 which was added in https://github.com/sorin-ionescu/prezto/pull/1376/

modules/command-not-found/README.md

@@ -1,15 +1,23 @@
 Command-Not-Found
 =================
 
-Displays installation information for not found commands by loading the
-[command-not-found][1] tool on Debian-based and Arch Linux-based distributions.
+When you try to use a command that is not available locally, searches
+the package manager for a package offering that command and suggests
+the proper install command. 
+
+Debian-based and Arch Linux-based distributions use the [`command-not-found`][1] tool.
+
+macOS uses Homebrew's [`command-not-found` clone][2]. Note that you also need to [follow the instructions to tap the `command-not-found` homebrew repository][3].
+
 
 Authors
 -------
 
-*The authors of this module should be contacted via the [issue tracker][2].*
+*The authors of this module should be contacted via the [issue tracker][4].*
 
   - [Joseph Booker](https://github.com/sargas)
 
 [1]: https://code.launchpad.net/command-not-found
-[2]: https://github.com/sorin-ionescu/prezto/issues
+[2]: https://github.com/Homebrew/homebrew-command-not-found
+[3]: https://github.com/Homebrew/homebrew-command-not-found#install
+[4]: https://github.com/sorin-ionescu/prezto/issues