docs: add CONTRIBUTING.md with setup, tests, and claim flow (Fixes #18)

[Thalassa09]

Fixes #18. Added CONTRIBUTING.md with setup, test instructions, and claim workflow, and linked it in README.md.

[KTQ0912]

QA Verification Report — PlantGuide PR #374 (CONTRIBUTING.md + README link, Fixes #18)

Target

• PR: docs: add CONTRIBUTING.md with setup, tests, and claim flow (Fixes #18) #374
• Author: @Ardhiwaluyo
• Head commit SHA: 9ef0b34
• Bounty scope: [25 MRG] CONTRIBUTING.md + good-first-issue path #18 (contributing docs)
• Verifier: @KTQ0912 (QA bounty [25 MRG] Species pack: Alocasia Zebrina (alocasia_zebrina) — photo + care card #64)

Commands run and results

cd PlantGuide
git checkout pr374 (9ef0b34)
python -m pytest tests/ -q

Result: 49 passed, 19.4s. No failures.

Manual test summary

• Adds CONTRIBUTING.md with development setup (clone, venv, editable install), test/lint commands (pytest, ruff), and MergeOS bounty claim process; links it from README.md.
• Docs-only change; content accurate against the repo's actual tooling (pytest + ruff + pyproject dev extras verified present).

Evidence status

Provided and complete: docs accurate, full suite passes, no code churn.

Final recommendation

APPROVE — ready for maintainer review / merge.