Initial version of contributing guide

Author: JonathanHuot

docs/contributing.rst

@@ -0,0 +1,54 @@
+============
+Contributing
+============
+
+Test simple changes
+===================
+
+Requests-OAuthlib is using `tox`_ as main test tool.
+It helps creating the required virtualenv for your python version.
+For example, if you have installed Python3.7:
+
+.. sourcecode:: bash
+
+   $ tox -e py37
+
+
+Validate documentation changes
+==============================
+
+Tox contains also a build method to generate documentation locally.
+
+.. sourcecode:: bash
+
+   $ tox -e docs,readme
+
+Then open the HTML page in `_build/html/index.html`
+   
+
+Verify all pythons versions
+===========================
+
+Requests-OAuthlib supports multiple versions of Python.
+You can test all Python versions conveniently using `tox`_.
+
+.. sourcecode:: bash
+
+   $ tox
+
+In order to run successfully, you will need all versions of Python installed. We recommend using `pyenv`_ to install those Python versions.
+
+.. sourcecode:: bash
+
+   $ pyenv install 2.7.18
+   $ pyenv install 3.4.10
+   $ pyenv install 3.5.10
+   $ pyenv install 3.6.14
+   $ pyenv install 3.7.11
+   $ pyenv install pypy2.7-7.1.1
+   $ pyenv install pypy3.6-7.1.1
+
+.. _`tox`: https://tox.readthedocs.io/en/latest/install.html
+.. _`virtualenv`: https://virtualenv.pypa.io/en/latest/installation/
+.. _`pyenv`: https://github.com/pyenv/pyenv
+

docs/index.rst

@@ -72,6 +72,7 @@ Getting Started:
    examples/examples
 
    api
+   contributing