Write policy for AI usage

Misc notes (to be revised / think through):

• mention which github issues are considered open for grabs (eg, if an issue is already assigned, no PR will be considered for it)
• contributors should mention AI usage and how it was used. mention that by opening an issue or submitting a PR, the human behind takes all responsibility. Mention that while AI usage is OK, the human is asked to review everything before submitting.
• repeated offenders that don't follow these guidelines will be blocked.
• clear attempts to "contributions farming" are against the policy and PRs will be closed.