Add AI Engineering Cheatsheet contribution standards - #21

What changed

• Added the repository's root CONTRIBUTING.md using the complete contribution workflow and workflow-pack rules.
• Added a dedicated Contributing to the AI Engineering Cheatsheet section.
• Corrected the workflow-pack terminology table so it renders properly.

AI Engineering Cheatsheet standards

The new rules require:

• one focused guide per pull request;
• genuine, repeated relevance to engineers building or operating production LLM systems;
• actionable decisions, checks, controls, failure modes, or examples;
• important disadvantages and limitations—not one-sided recommendations;
• original synthesis with direct credit and license checks for materially used sources;
• manual verification of claims, links, examples, and diagrams;
• removal of AI-generated filler, duplicates, unsupported numbers, and fake citations;
• an entry in AI Engineering Cheatsheet/README.md for every accepted guide.

Why

The series should grow through useful production-engineering guidance, not generic AI summaries, copied social posts, prompt collections, or low-effort LLM-generated lists.

Validation

• Confirmed the branch adds only CONTRIBUTING.md.
• Confirmed the AI Cheatsheet section includes scope, exclusions, source-credit rules, quality standards, and a pull request checklist.
• Confirmed the root guide retains the Apache License 2.0 contribution notice.
• Confirmed Markdown fences are balanced and no trailing whitespace or merge markers are present.