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
+https://github.com/sfstoolbox/sfs-matlab/.
+Contributions are always welcome!
+
+Building the Documentation
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If you make changes to the documentation, you can re-create the HTML pages
+using Sphinx_.
+You can install it and a few other necessary packages with::
+
+   python3 -m pip install -r doc/requirements.txt --user
+
+To create the HTML pages, use::
+
+   sphinx-build -b html -d ./_build/doctrees . ./_build/html-preview/
+
+The generated files will be available in the directory
+``build/sphinx/html-preview/``.
+
+.. _Sphinx: http://sphinx-doc.org/
+
+Creating a New Release
+^^^^^^^^^^^^^^^^^^^^^^
+
+New releases are made using the following steps:
+
+#. Bump version number in ``SFS_version.m``
+#. Update ``NEWS``
+#. Commit those changes as "Release x.y.z"
+#. Create an (annotated) tag with ``git tag -a x.y.z``
+#. Check that all author details in ``.zenodo.json`` are correct
+#. Push the commit and the tag to Github and `add release notes`_ containing a
+   link to the documentation with http://matlab.sfstoolbox.org/en/x.y.z and the
+   bullet points from ``NEWS``
+#. Check that the new release was built correctly on RTD_, delete the "stable"
+   version and select the new release as default version
+#. Check that the new release was registered at Zenodo_ and edit the release
+   notes on Github to include the DOI badge
+
+.. _add release notes: https://github.com/sfstoolbox/sfs-matlab/tags
+.. _RTD: http://readthedocs.org/projects/sfs-matlab/builds/
+.. _Zenodo: https://zenodo.org/search?page=1&size=20&q=sound%20field%20synthesis%20toolbox&sort=bestmatch