Added info on how to create the virtual env.

docs/developer-guide.md

@@ -57,6 +57,12 @@ Ensure you have following other projects up and running:
 - engine-api
 - engine-clock (optional)
 
+Create an virtual environment allowing you to ensure having the proper Python version and dependencies:
+
+```shell
+~/code/aura/engine$ virtualenv -p python3.8 python
+```
+
 Then you can start the engine. The following command includes the activation of your virtual environment, which you have created during the installation step (`source python/bin/activate`):
 
 ```shell