add link to python virtual env

doc.zih.tu-dresden.de/docs/software/machine_learning.md

@@ -46,7 +46,7 @@ marie@romeo$ module load modenv/scs5
 
 ### Python and Virtual Environments
 
-Python users should use a virtual environment when conducting machine learning tasks via console.
+Python users should use a [virtual environment](python_virtual_environments.md) when conducting machine learning tasks via console.
 In case of using [sbatch files](../jobs_and_resources/batch_systems.md) to send your job you usually
 don't need a virtual environment.