docs: add backend contributing guide

[cuph7022]

Summary

• add CONTRIBUTING.md covering branch naming, commit style, local setup, checks, testing, CI expectations, and security notes
• link the guide from README.md

Closes #291

Validation

• Reviewed package scripts, README, and CI workflow via GitHub API.
• Not run locally: docs-only change; no dependencies were downloaded or executed on the contributor machine.

[mikewheeleer]

good stuff, reads well and covers the important bits. in it goes