[Docs] Add CHANGELOG.md entry conventions to CONTRIBUTING.md

[davidmaronio]

Context

CHANGELOG.md exists and is updated over time but CONTRIBUTING.md gives no guidance on the expected format or when a PR should add an entry, leading to inconsistent changelog hygiene.

Acceptance criteria

• Behavior/gap described above is addressed or confirmed and documented
• Relevant unit test(s) added or updated
• cargo fmt --check and cargo clippy --all-targets --features testutils -- -D warnings pass

Files to touch

CONTRIBUTING.md, CHANGELOG.md

Out of scope

• Unrelated refactors outside the files listed above
• Changes to unrelated contract functions not mentioned in this issue