Outputs the merged pull requests on Homebrew/brew between two Git refs.

Write the README but also a `.travis.yml` file (and in future perhaps a
`Jenkinsfile`).

These are the ones that are auto-installed without tapping and it makes
sense to make them more prominent in our documentation as we've
offloaded various functionality over the years to each of them.

This option is used to test updating between tags.

As requested in #1199 this updates the documentation accordingly for
that change.

Add some missed changes. I'll add a CI check for this at a future point.