Add a CONTRIBUTING file

This file will be displayed to contributors when they create a pull request in any branch (hence why it is not written in a branch-specific way).

.github/CONTRIBUTING.rst

+Contributing to Python
+======================
+
+Build Status
+------------
+
+- master
+
+  + `Stable buildbots <http://buildbot.python.org/3.x.stable/>`_
+
+- 3.6
+
+  + `Stable buildbots <http://buildbot.python.org/3.6.stable/>`_
+
+- 3.5
+
+  + `Stable buildbots <http://buildbot.python.org/3.5.stable/>`_
+
+- 2.7
+
+  + `Stable buildbots <http://buildbot.python.org/2.7.stable/>`_
+
+
+Contribution Guidelines
+-----------------------
+Please read the `devguide <https://cpython-devguide.readthedocs.io/>`_ for
+guidance on how to contribute to this project. The documentation covers
+everything from how to build the code to submitting a pull request. There are
+also suggestions on how you can most effectively help the project.
+
+Please be aware that our workflow does deviate slightly from the typical GitHub
+project. Issues are tracked at https://bugs.python.org/ and should be mentioned
+in pull requests using the format of ``bpo-12345`` . If your pull request is in
+response to a specific issue then please mention it in the title,
+e.g. “bpo-12345: fix comment in tokenizer.c”.
+
+If you are making a code contribution or large documentation contribution,
+please feel free to add yourself to the ``Misc/ACKS`` file alphabetically.
+
+
+Code of Conduct
+---------------
+All interactions for this project are covered by the
+`PSF Code of Conduct <https://www.python.org/psf/codeofconduct/>`_. Everyone is
+expected to be open, considerate, and respectful of others no matter their
+position within the project.