Document new Git workflow in CHANGELOG.md

[thinkingserious]

Issue Summary

We want to update our git workflow to function more like Gitflow. We need to update the CONTRIBUTING.md file and PULL_REQUEST_TEMPLATE to direct contributors to fork/branch off the development branch and to merge their PRs with the development branch. Once this issue is complete, we will be creating the development branch and making that branch the default. This is part of a larger strategy to execute releases for this SDK on a predictable cadence.

Acceptance Criteria

• Update CONTRIBUTING.md documentation, explaining the process of forking and branching off the development branch and submitting PRs agains the development branch.
• Update PULL_REQUEST_TEMPLATE with the same addition to the CONTRIBUTING.md documentation.

[childish-sambino]

Closing all open issues and PRs. This repo is no longer maintained. Please use https://github.com/sendgrid/sendgrid-nodejs/tree/master/packages/client instead.