Update CONTRIBUTING.md

[MikeRalphson]

• [ ] Guidance on adding a linter new rule-action (if none of the existing ones can perform the action, or be extended)
• [ ] Expand on guidelines re: keeping commits to logical areas of the code / individual packages where possible
• [ ] Use of conventional-commit standard going forward
• [x] Advice not to include any package-lock.json changes PRs, for security reasons. File will be regenerated by a maintainer if necessary. See here for an example