Add a CONTRIBUTING.md with the contract conventions, append-only error policy, and PR checklist

[mikewheeleer]

Document contributor conventions and the PR checklist

Description

The repository has no CONTRIBUTING.md, yet src/lib.rs encodes several non-obvious conventions a contributor must follow to get a PR merged: the append-only RouterError numbering rule ("never reuse or renumber a variant once it has shipped"), the symbol_short! ≤ 9-character event-topic constraint, the admin-auth + pause-gate patterns, and the fmt/build/test commands. New contributors rediscover these by trial and error. This issue captures them in a single contributor guide.

Requirements and context

• Repository scope: StableRoute-Org/Stableroute-contracts only.
• Create CONTRIBUTING.md covering: the append-only error policy, the event-topic naming/length rule, the admin-auth and pause-gate patterns, the storage-tier/TTL conventions, and the local cargo fmt --all -- --check / cargo build / cargo test workflow.
• Include a PR checklist (tests added, NatSpec on new entrypoints, no error renumbering, events asserted, docs updated).
• Link the StableRoute Discord: https://discord.gg/37aCpusvx
• No production code change.

Suggested execution

• Fork the repo and create a branch
• git checkout -b docs/contracts-45-contributing-guide
• Implement changes
  • Write code in: src/lib.rs — no behavior change; optionally cross-link conventions in a top-of-file comment.
  • Write comprehensive tests in: src/lib.rs #[cfg(test)] mod test — no new tests required; ensure existing ones pass.
  • Add documentation: create CONTRIBUTING.md and link it from README.md.
  • Include NatSpec-style doc comments (///) consistency note in the guide.
  • Validate security assumptions: the documented conventions match the actual enforced patterns in the source.
• Test and commit

Test and commit

• Run cargo fmt --all -- --check, cargo build, and cargo test.
• Cover edge cases and failure paths: verify every documented convention maps to a real pattern in src/lib.rs.
• Include the full cargo test output and a short security notes section in the PR description (threat model + mitigations).

Example commit message

docs: add CONTRIBUTING guide with conventions and PR checklist

Guidelines

• Minimum 95 percent test coverage for impacted modules.
• Clear, reviewer-focused documentation.
• Timeframe: 96 hours.

Community & contribution rewards

• 💬 Join the StableRoute community on Discord for questions, reviews, and faster merges: https://discord.gg/37aCpusvx
• ⭐ This is a GrantFox OSS / Official Campaign task and may be rewarded. When your PR is merged you'll be prompted to rate the project — if this issue and the maintainers helped you ship, we'd be grateful for a 5-star rating. Clear questions in Discord and tidy, well-tested PRs are the fastest path to a merge and a reward.

[Baskarayelu]

I’d love to be assigned this issue and contribute across the relevant areas, whether it involves smart contracts, UI development, backend logic, design implementation, or overall system integration. I’m comfortable working across the full stack and can adapt to the project’s technical requirements to ensure a smooth and consistent contribution.