Write dev/contributor docs

[seanmonstar]

• Update CONTRIBUTING.md
  • Move most content into dev folder documents
• Make docs folder with developer docs
  • README as an index file
  • Charter and Tenets
  • Conduct
  • Roadmap
  • Issues, labels
  • Pull requests
  • Committing
  • Testing
  • MSRV (Minimum Supported Rust Version)
  • Release Process
  • Design
    • Feature flags
    • private-in-public types as impl Trait
    • Poll functions vs async
  • Governance
• Render dev guide on https://hyper.rs/contrib