Restore caveat to use Homebrew Zsh as login shell

I think this single caveat should be in place.

`chsh -s /usr/local/bin/zsh` (to change default user login shell) will not work if Homebrew Zsh is not added to `/etc/shells`

If you're absolutely against editing `/etc/shells`, user should be advised to update their login shell through OS X prefs: Users & Groups > Advanced Options > Login shell (Advanced Options are accessed by secondary clicking user you would like to edit, and can only be accessed if the pref panel is unlocked)