Mention homebrew-binary in contributing docs

There have been a lot of cli-only app submissions, and it’s clear that few know of [homebrew-binary](https://github.com/Homebrew/homebrew-binary). [As suggested](https://github.com/caskroom/homebrew-cask/pull/3891#issuecomment-42219305), this documents and clarifies cases when a *cask* should be submitted first to *hombrew-binary*.