Add `brew cask` install instructions for macOS
Add `brew cask` install instructions for macOS

I added a formula for `aws-vault` to homebrew cask in https://github.com/caskroom/homebrew-cask/pull/23637. I _propose_ that the README should talk about that as an installation method since many folks just aren't used to or comfortable with manually dropping binaries in place, setting permissions correctly, etc.

I'm not wedded to this at all, though, so feel free to close if you disagree :smile:
99designs/aws-vault,99designs/aws-vault