Add a contributing.rst document stub to describe precommit and general workflow

Author: gnikonorov

CONTRIBUTING.rst

@@ -0,0 +1,39 @@
+============================
+Contribution getting started
+============================
+
+Contributions are highly welcomed and appreciated. Every little bit of help counts,
+so do not hesitate!
+
+.. contents::
+   :depth: 2
+   :backlinks: none
+
+
+Preparing Pull Requests
+-----------------------
+
+#. Fork the repository.
+
+#. Enable and install `pre-commit <https://pre-commit.com>`_ to ensure style-guides and code checks are followed::
+
+   $ pip install --user pre-commit
+   $ pre-commit install
+
+   Afterwards ``pre-commit`` will run whenever you commit.
+
+   https://pre-commit.com/ is a framework for managing and maintaining multi-language pre-commit hooks
+   to ensure code-style and code formatting is consistent.
+
+#. Install `tox <https://tox.readthedocs.io/en/latest/>`_::
+
+   Tox is used to run all the tests and will automatically setup virtualenvs
+   to run the tests in.
+   (will implicitly use http://www.virtualenv.org/en/latest/)::
+
+    $ pip install tox
+    $ tox -e linting,py37
+
+#. Follow **PEP-8** for naming and `black <https://github.com/psf/black>`_ for formatting.
+
+#. Add a line item to the current **unreleased** verrsion in ``CHANGES.rst``, unless the change is trivial.