Add Support Tiers documentation

This provides clarify to users about the support tiers for Homebrew.

Co-authored-by: Douglas Eichelberger <697964+dduugg@users.noreply.github.com>
Co-authored-by: Bo Anderson <mail@boanderson.me>
Co-authored-by: Sam Ford <1584702+samford@users.noreply.github.com>