doc: Add CONTRIBUTING.md

Closes #2675
Closes #2668

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+# Contributing to Tornado
+
+## The basics
+
+* We use `black` as code formatter and recommend configuring your editor to run this automatically
+  (using the version specified in `requirements.txt`). Commits that are not properly formatted
+  by `black` will be rejected in CI.
+* Before committing, it is recommended to run `tox -e lint,docs,py3`. This will verify that the
+  code is formatted correctly, type checking with `mypy` passes, the `sphinx` build for docs has
+  no errors, and the main test suite passes with the current version of python.
+* Nearly all code changes should have new or updated tests covering the changed behavior.
+  Red/green TDD is encouraged.
+
+## Testing
+
+* We use `tox` as a test runner to run tests in various configurations with the correct
+  dependencies. `tox -e py3` runs most of the tests, while `tox -e py3-full` can be used
+  to run a more extensive version of the test suite which as extra dependencies. The
+  `-full` configurations are necessary when working on certain modules, including
+  `curl_httpclient.py`, `twisted.py`, or `pycares.py`.
+* The fastest way to run the tests is to bypass `tox` and run `python3 -m tornado.test`.
+  To run a subset of tests, add a module, class, or method name to the command line:
+  `python3 -m tornado.test.httputil_test`.
+* Tests can also be run with the standard library's `unittest` package CLI. This is useful
+  for integration with some editors.
+* Tornado does not use `pytest`. Some effort has been made to make the tests work with 
+  the `pytest` runner, but this is not maintained.
+
+## Documentation
+
+We use Sphinx with the `autodoc` extension to build our docs. To build the docs run
+`tox -e docs` and find the output in `./.tox/docs/tmp/html/index.html`
+
+## AI policy
+
+Tornado has a neutral stance towards AI-generated code. All pull requests, whether human
+or machine-generated, are subject to strict code review standards. However, PRs that appear
+to be AI-generated *and* contain clear flaws (such as failing CI) may be closed without
+detailed review. 
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