Add CONTRIBUTING file

CONTRIBUTING.rst

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+Contributing
+------------
+
+If you find errors, omissions, inconsistencies or other things that need
+improvement, please create an issue or a pull request at
+http://github.com/hagenw/sphinxcontrib-katex/.
+Contributions are always welcome!
+
+Instead of pip-installing the latest release from PyPI, you should get the
+newest development version from Github_::
+
+   git clone https://github.com/hagenw/sphinxcontrib-katex.git
+   cd sphinxcontrib-katex
+   python setup.py develop --user
+
+.. _Github: http://github.com/hagenw/sphinxcontrib-katex/
+
+This way, your installation always stays up-to-date, even if you pull new
+changes from the Github repository.
+
+If you prefer, you can also replace the last command with::
+
+   pip install --user -e .
+
+... where ``-e`` stands for ``--editable``.
+
+
+New releases are made using the following steps:
+
+#. Bump version number in ``sphinxcontrib/katex.py``
+#. Update ``NEWS.rst``
+#. Commit those changes as "Release x.y.z"
+#. Create an (annotated) tag with ``git tag -a x.y.z``
+#. Clear the ``dist/`` directory
+#. Create a source distribution with ``python3 setup.py sdist``
+#. Create a wheel distribution with ``python3 setup.py bdist_wheel --universal``
+#. Check that both files have the correct content
+#. Upload them to PyPI with twine_: ``twine upload dist/*``
+#. Push the commit and the tag to Github and `add release notes`_ containing a
+   link to PyPI and the bullet points from ``NEWS.rst``
+
+.. _twine: https://pypi.python.org/pypi/twine
+.. _add release notes: https://github.com/hagenw/sphinxcontrib-katex/tags