docs: add CONTRIBUTING guide for error-code and event conventions

[Jagadeeshftw]

Summary

Adds CONTRIBUTING.md documenting the conventions that keep the escrow contract's ABI stable:

• Append-only error codes — never renumber or reuse, with the current EscrowError table reproduced exactly (codes 1..=12, reflecting that code 2 RequestsMustBePositive is also reused for a negative price). Next code must be 13.
• symbol_short! ≤ 9-char event-topic constraint.
• Additive-only events stance, with the current usage / settled / paused payload shapes documented.
• unwrap_or getter-default convention (and Option when absence is meaningful).
• Test panic-assertion style — #[should_panic(expected = "Error(Contract, #N)")] — plus event/state assertion guidance.
• The cargo fmt / cargo build / cargo test gate, and the 95%-coverage / 96-hour campaign expectations.

Linked from README.md. No code changed.

Closes #46

Test output (gate, unchanged code)

test result: ok. 23 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.21s

Security notes: Docs-only change; no contract code or dependencies are modified. The error-code table is reproduced verbatim from lib.rs so the documented ABI matches the implementation.

[mikewheeleer]

good — documenting the error-code and event conventions will help every future PR. lgtm 🙌