introduce contributing doc + main branch convention (#84)

docs/fragments/virtualenv_explainer.rst

@@ -0,0 +1,25 @@
+**Optional:** Often, the best way to guarantee a clean Python 3 environment is with
+`virtualenv <https://virtualenv.pypa.io/en/stable/>`_. If we don't have ``virtualenv`` installed
+already, we first need to install it via pip.
+
+.. code:: sh
+
+  pip install virtualenv
+
+Then, we can initialize a new virtual environment ``venv``, like:
+
+.. code:: sh
+
+  virtualenv -p python3 venv
+
+This creates a new directory ``venv`` where packages are installed isolated from any other global
+packages.
+
+To activate the virtual directory we have to *source* it
+
+.. code:: sh
+
+  . venv/bin/activate
+
+  # and when leaving this context:
+  deactivate