Update readme for Mac OS

README.md

@@ -10,11 +10,15 @@ as soon as possible. See https://github.com/minad/mimemagic/issues/97 for detail
 Dependencies
 ============
 
-You will require a copy of the Freedesktop.org shared-mime-types database to be available. If you're on Linux,
+You will require a copy of the Freedesktop.org shared-mime-info database to be available. If you're on Linux,
 it's probably available via your package manager, and will probably be in the location it's being looked for
 when the gem is installed.
 
-If you're not on Linux, or for whatever reason the file is found, you'll need to obtain a copy from the Internet.
+macOS users can install via Homebrew with `brew install shared-mime-info`.
+
+If for whatever reason you can't do either of those things, you'll need to obtain a copy from the Internet. Set
+the environment variable `FREEDESKTOP_MIME_TYPES_PATH` before installing the gem in order to point the build at
+that location.
 
 (TODO: Locate a reliable source for the compiled file, and link to it both here, and in the error message if
 one isn't found.)