Doc: Define policy on AI usage in OpenTTD development (#14537)

CONTRIBUTING.md

@@ -196,6 +196,29 @@ These include:
 You may also want the guide to [compiling OpenTTD](./COMPILING.md).
 
 
+## Use of AI
+
+OpenTTD is a labour of love, created by people.
+
+Please refrain from submitting issues or pull requests that have been generated by an LLM or other fully-automated tools.
+Any submission that is in violation of this policy will be closed, and the submitter may be blocked from this repository without warning.
+
+If you submit an issue, you need to understand what your issue description is saying.
+You need to be able to answer questions about your bug report or feature request.
+Using an AI tool to _proofread_ your issue/comment text is acceptable. Using an AI tool to _write_ your issue/comment text is not.
+
+If you submit a pull request, you need to understand what every line of code you've changed does.
+If you can't explain why your PR is doing something, then do not submit it.
+Using an AI tool to generate entire lines of code is unacceptable.
+
+The rationale behind this policy is that automated contributions are a waste of the maintainers' time.
+Humans spend their time and brainpower reviewing every submission.
+Issues or pull requests generated by automation tools create an imbalance of effort between the submitter and the reviewer.
+Nobody learns anything when a maintainer reviews code written by an LLM.
+
+Additionally, AI-generated code conflicts with this project's license (GPL v2), since you cannot truly release code for use if you didn't author it yourself.
+
+
 ## Project goals
 
 ### What are the goals of the official branch?