docs: add contributing guide with branch, testing, and a11y conventions

[cuph7022]

Summary

• add a dedicated CONTRIBUTING.md for the AgentPay frontend workflow
• document branch naming, local checks, UI/a11y conventions, API helper usage, PR checklist, and security notes
• link the new guide from README.md

Closes #29

Validation

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

[mikewheeleer]

this is really clean and well organised, thanks for writing it up. merging 🙌