add to `brew bundle` example and add a note about `set -e` in scripts

## Contents

- Clarifies that `./` is not necessary as a path prefix to specified file path.
- Updates list of `brew bundle` examples (further assurance that you any brew command can be added).
- Adds a note about `set -e` in scripts that wrap `brew bundle`.

## Rationale

The reason I opened this PR is because I wrote [something similar][ontap] to `brew bundle` before I knew about `brew bundle`. I wanted to convert my script to a homebrew external command; but when I went searching for how to do it, I found this.

My script is more complicated because it handles the errors that `brew` throws and instead throws a warning instead. Without this, any script that uses `set -e` will fail. Also, I like the approach taken by `brew bundle` because it is simpler (and faster) -- the trade-off (and it is a fair one) is that you'll have to wrap the call to `brew bundle` with a pair of `set +e` and `set -e`.

Anyway, I'll be using `brew bundle` going forward.

[ontap]: https://github.com/wilmoore/ontap