Add draft contributing doc.

CONTRIBUTING.rst

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+Contributor guide
+=================
+
+Contributions are welcome, and several things about this repository
+have been set up to make the process easier for everyone (including
+you!).
+
+
+Dev environment recommendations
+-------------------------------
+
+* Please use a code editor/IDE that supports `EditorConfig
+  <https://editorconfig.org>`_ Most editors do nowadays, so you
+  probably don't have to worry about it, but it will help to
+  automatically apply some formatting and style rules.
+
+* Please make sure you have `pre-commit <https://pre-commit.com>`_
+  installed, and in your local checkout of this repository run
+  ``pre-commit install`` to set up the pre-commit hooks.
+
+* Please have `nox <https://nox.thea.codes/en/stable/>`_ installed on
+  your computer, and run at least the unit-test suite (``python -m nox
+  -t tests``) before you push your commits to GitHub. If you can
+  manage it, installing the full set of supported Python versions and
+  running the entire suite (``python -m nox``) is even better. Tools
+  like `pyenv <https://github.com/pyenv/pyenv>`_ or ``asdf
+  <https://asdf-vm.com>`_ can help with installing and managing
+  multiple Python versions.
+
+Following these steps will catch a lot of potential problems for you,
+and can even fix several of them automatically.
+
+
+Code style
+----------
+
+The pre-commit hooks will auto-format code with `isort
+<https://pycqa.github.io/isort/>`_ and `Black
+<https://black.readthedocs.io/>`_. Most editors and IDEs also support
+auto-formatting with these tools every time you save a file. The CI
+suite will disallow any code that does not follow the isort/Black
+format.
+
+All code must also be compatible with all versions of Python currently
+supported by the Python core team. See `the Python dev guide
+<https://devguide.python.org/versions/>`_ for a current chart of
+supported versions.
+
+
+Other guidelines
+----------------
+
+* If you need to add a new file of code, please make sure to put a
+  license identifier comment on the first line after the file's
+  docstring. You can copy and paste the license identifier comment
+  from any existing file, where it looks like this: ``#
+  SPDX-License-Identifier: BSD-3-Clause``
+
+* Documentation and tests are not just recommended -- they're
+  required. Any new file, class, method or function must have a
+  docstring and must either include that docstring (via autodoc) in
+  the built documentation, or must have manually-written documentation
+  in the ``docs/`` directory. Any new feature or bugfix must have
+  sufficient tests to prove that it works, and the test coverage
+  report must come out at 100%. The CI suite will fail if test
+  coverage is below 100%, if there's any code which doesn't have a
+  docstring, or if there are any misspelled words in the documentation
+  (and if there's a word the spell-checker should learn to recognize,
+  add it to ``docs/spelling_wordlist.txt``).
+
+* Other than the tests in the special ``tests/end_to_end.py`` file,
+  tests for this module must not make real requests to the Akismet web
+  service and must not require a valid Akismet API key -- see the
+  existing test files for examples of how to write tests that use mock
+  responses.