Import documentation for `brew bundle`

This was in the (soon to be archived) Homebrew/homebrew-bundle
repository's `README`. There's some good content here that doesn't
cleanly fit into the `brew` manpage so let's move it here.

While we're here, let's de-emphasize `whalebrew` as it's not
widely used and add commented-out deprecation warnings for its future
deprecation.