Add Danger for automated code review conventions

[bchrobot]

See Danger's Getting Started

This should:

• Enforce updates to change log for all commits
• Enforce code coverage percentage does not decrease
• Suggest updates to readme and/or docs/

[bchrobot]

I started looking into this again and ran up against the same wall as before. CircleCI does not allow filtering jobs based on pull request. Danger cannot work correctly if it is run on a commit unrelated to a PR.

You can enable "only build pull requests" on CircleCI to get around this, but that precludes having two "main" branches (develop and master) for which all commits are still run. There is a stale CircleCI thread about this.