Explain about bash completion on macOS

[klauer]

macOS-provided bash is bizarrely ancient. zsh is their preferred replacement, which is now default.

To get a newer bash, we need to fall back to homebrew:

1. Install homebrew (enable it in bash_profile)
2. Install bash and bash-completion via brew install bash bash-completion
3. Old path was [ -f /usr/local/etc/bash_completion ] && . /usr/local/etc/bash_completion new path is somewhere in /opt/homebrew I think