Add/community files

[jeffpaul]

Description of the Change

This PR adds a Code of Conduct file per #137 as well as CHANGELOG.md and CREDITS.md files to more closely align to 10up's Open Source Best Practices. It also updates the README.md file with references to the newly created files, adds a Table of Contents, and other minor updates.

Alternate Designs

Keep as-is or cherry-pick desired changes specific for this repo.

Benefits

Ensures this project follows repo standards akin to other 10up projects.

Possible Drawbacks

None identified.

Verification Process

Manually verified via GitHub Desktop UI.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

Resolves #137.

Changelog Entry

### Added
- Documentation updates

[jeffpaul]

@ericmann note that we're also working on renaming the master branch to trunk across 10up repos (see 10up/Open-Source-Best-Practices#62). If you're good with it, I can make those changes and update this PR accordingly?

[coveralls]

Coverage remained the same at 52.367% when pulling cd0bfb0 on add/community-files into 9927f0c on master.

[ericmann]

@jeffpaul Perfectly ok with a change to trunk and I just made that. Feel free to update any other branches to fit the new pattern and I'll jettison the master branch later.