Additional tasks for README.md and CONTRIBUTING.md

[mrossesf]

If #215 gets merged, there's some additional work to complete still:

General keystroke and copy edit, plus in CONTRIBUTING.md:

• [x] Who can contribute?
• [ ] Code of Conduct
• [ ] Find a task (needs more details per questions in #133 )
• [x] Set up the project
• [x] Fix or improve content
  • [x] Get your flow, yo
  • [ ] Add or edit Markdown files
  • [x] Add or edit code comments and generate Markdown files
• [ ] Create a pull request (needs a copy edit)
  • [ ] Get a PR reviewed
  • [ ] Resolve merge conflicts