Auto-generate the CHANGELOG.md file upon release

[thinkingserious]

Issue Summary

For every release, we currently hand craft a CHANGELOG.md update, now we would like to automate this process. Please see the existing CHANGELOG.md for formatting structure.

Acceptance Criteria

• A script, which when run, creates a CHANGELOG.md update based on any merged PRs since the last release.

[mauricetmeyer]

I can take on this one too

Edit: Progress can be seen at sendgrid/php-http-client#115 (comment)

[childish-sambino]

Closing as we're now using internal tooling to automate this.