[Savonitar]

Purpose of the change

The flink-benchmarks repository has no PR template. Add one so contributors can provide review context and disclose AI assistance as described in the Flink contribution guide.

Addresses FLINK-40764, part of FLINK-40749.

Brief change log

• Add concise prompts for purpose, affected benchmarks, verification, and impact.
• Reflect the README guidance to include benchmark results, with relevant checks for maintenance changes.
• Include the current Flink AI-disclosure section, linking the contribution guide instead of an absent AGENTS.md.

Verifying this change

Checked the complete one-file diff, whitespace, Apache license header, and HTML comment boundaries. Confirmed that the AI checkbox and Generated-by placeholder remain visible outside comments. No Maven build or benchmarks were run for this Markdown-only change.

Impact and documentation

Adds contribution guidance only. No changes to benchmarks, dependencies, or result comparability.

---

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Generated-by: OpenAI Codex (GPT-6)