Create a Contributing Guide

[egillespie]

Write a Contributing Guide in the same detail and style as www/.github/CONTRIBUTING.md.

Acceptance Criteria

• The README.md links to the Contributing Guide the same way that www/README.md does
• The guide is located at .github/CONTRIBUTING.md
• The guide includes a table of contents
• The guide includes a link to the Code of Conduct
• The guide includes Issue Reporting Guidelines
• The guide includes Pull Request Guidelines
• The guide shows Development Setup that includes recommended tools for a good dev experience
• The guide describes how to make changes or add to the web font (move "Making changes" from README.md to the Contributing Guide)
• The guide explains how to use the web font (move "Using the web font" from README.md to the Contributing Guide)