Add first draft for tests doc

docs/dev_guide/tests.rst

 Tests
-==================================
\ No newline at end of file
+==================================
+
+Tests should run using tox_
+
+.. _tox: https://tox.readthedocs.io/en/latest/
+
+
+To install it with pip use:
+
+.. code-block:: bash
+    
+    $ pip install tox
+
+
+To start testing simply run:
+
+.. code-block:: bash
+
+    $ tox
+    ...
+    py37: commands succeeded
+    py38: commands succeeded
+