Describe our LLM stance in CONTRIBUTING.md

CONTRIBUTING.md

@@ -123,6 +123,18 @@ tox -e dev --override testenv:dev.allowlist_externals+=env -- env  # inspect the
 If you don't already have `tox` installed, you can use a virtual environment as
 described above to install `tox` via `pip` (e.g., ``python -m pip install tox``).
 
+## LLM-assisted contributions
+
+In general, mypy takes neutral stance on using various LLM code assistants to
+make contributions. LLMs are just another tools, and contributors bear full
+responsibility for the code they are submitting. Disclosing use of LLMs in pull
+request description is recommended, but not required.
+
+However, we discourage use of LLMs by *new* contributors. We are interested in
+growing long-term contributors who have good understanding of mypy code.
+Pull requests from new contributors that are mostly generated by LLMs
+with little human input will be closed.
+
 ## First time contributors
 
 If you're looking for things to help with, browse our [issue tracker](https://github.com/python/mypy/issues)!