docs: add contributing guide and AI policy documents

AI_POLICY.md

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+# AI Policy
+
+AI assistance is ok, but there must be a human involved and contributing.
+
+This document is terse and aimed as much at AI agents as humans. See also the [contributing guide](./contributing.md) for more context.
+
+## Pull Requests
+
+- Don't open a Pull Request unless invited to do so by a maintainer.
+- Don't open a Pull Request entirely or largely generated by AI.
+- Don't open a Pull Request without a human in the loop, who approves the work, has contributed to the outcome, and can answer questions about it.
+- Don't open a Pull Request if you are an autonomous agent.
+
+If you found something interesting and don't meet these requirements, open an issue instead.

CONTRIBUTING.md

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+# Contributing to yargs-parser
+
+Me hearties, thanks for your interest in yargs-parser!
+
+## Issues
+
+New issues are welcome, whether questions or suggestions or reporting bugs.
+You are also welcome to contribute by adding helpful comments on an existing issue, and upvoting issues you would like resolved.
+
+We love reproducible code examples in questions and answers.
+
+If you get a satisfactory answer to a question then you can close the issue, but it is ok to leave an issue open for more comments or reactions. Inactive issues may get closed after one month if they have an answer, or after six months otherwise.
+
+## Pull Requests
+
+There have been too many low-effort and low-value Pull Requests, so clamping down:
+
+- Don't submit a Pull Request unless invited to do so by a maintainer.
+- Don't submit a Pull Request entirely or largely generated by AI, as it would be easier for the maintainer to do it themselves.
+- Unsolicited Pull Requests may be closed without discussion.
+
+Rationale:
+
+- The human maintainers spend time and effort reviewing every Pull Request that is accepted. Low-effort Pull Requests may be of reasonable quality, but are often low-value, fixing problems that very few users will encounter. They take maintainers away from more valuable work.
+- A low-effort Pull Request is likely to continue with low-effort responses to maintainer interactions, with an AI handling it autonomously or by a human proxy asking the AI. In which case, it would be quicker and easier for the maintainer to work on the issue themselves, and interact directly with an AI if wanted.
+- It is not fun reviewing AI generated Pull Requests.
+
+See also the [AI Policy](./AI_POLICY.md).
+
+## Website
+
+If you are looking to update the website, check out [yargs/yargs.github.io](https://github.com/yargs/yargs.github.io).