Add instructions for using venv.

Author: drvinceknight

docs/tutorials/contributing/setting_up_the_environment.rst

@@ -4,12 +4,18 @@ Setting up the environment
 Installing all dependencies
 ---------------------------
 
-All testing dependencies can be installed by running::
+All dependencies can be installed by running::
 
   $ pip install -r requirements.txt
 
 It is recommended to do this using a virtual environment tool of your choice.
 
+For example, when using the virtual environment library :code:`venv`::
+
+  $ python -m venv axelrod_development
+  $ source axelrod_development/bin/activate
+  $ pip install -r requrirements
+
 The git workflow
 ----------------