Added comment indicating that running directly on OSX is unsupported

Brewfile

 # Run `brew bundle` to automatically install these packages
 # http://robots.thoughtbot.com/brewfile-a-gemfile-but-for-homebrew
+#
+# Note that running edx-platform on OSX directly is unsupported: this file
+# exists for development purposes only. The recommended way to run edx-platform
+# on OSX is to use devstack, which installs edx-platform on an Ubuntu VM.
+# See: https://github.com/edx/configuration/wiki/edX-Developer-Stack
 
 install readline
 install sqlite