build: document use of ares.cachix.org in the top-level README
build: document use of ares.cachix.org in the top-level README

urbit/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,urbit/urbit,urbit/urbit,urbit/urbit,urbit/urbit,ngzax/urbit