Document the Python version explicitly
The spec uses 2.x currently and is not compatible with Python 3.
@@ -41,7 +41,7 @@ specs and event schemas in this repository.
Preparation
-----------
-To use the scripts, it is best to create a Python virtualenv as follows::
+To use the scripts, it is best to create a Python 2.x virtualenv as follows::
virtualenv env
env/bin/pip install -r scripts/requirements.txt
