Document the installation of completions in macOS

[jvican]

The installation steps for macOS does not document how to enable completions for bash so bash users are missing out on completions at the moment. Users of zsh (with oh-my-zsh) and fish have the completions enabled by default, but this is not the case for the rest of the users.

We should:

1. Document what users need to have completions working, see https://docs.brew.sh/Shell-Completion
2. Maybe make the homebrew recipe depend on bash-completion?

The issue was first reported by @tpolecat in https://gitter.im/scalacenter/bloop?at=5c2cfda42863d8612bca13d6, there's a fix available in the gitter channel to get bash completions working