Restore caveat on how 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)

Not sure if you would want to include in-depth instructions for either approach. Please advise and I'll send over another commit.