Create contribution guidelines md, asciidoc, or wiki

[Nikoraito]

Write up a document describing the consensus workflow for contributing to this project:

• Workflow overview
  • Making a branch on our remote
  • Install pnpm and running pnpm i
  • Making semantic commits (list of types, scopes?)
  • Making PRs
  • Getting code reviewed and merged
• Details
  • Detailed setup guide
  • Code style guidelines
  • Interface design and accessibility guidelines
  • Code review checklist

[Nikoraito]

fwiw most of this is in the readme already—if we add the design and codestyle sections, as well as the acceptance criteria for PRs, the readme will satisfy this.