[ci-skip] update readme.md

README.md

@@ -12,6 +12,7 @@ The following steps can be followed to set up a local environment:
 2. Create a new Python virtual environment
 
     ```bash
+    sudo apt update && sudo apt install -y virtualenv
     git clone git@gitlab.software.geant.org:nmaas/nmaas-docs.git
     cd nmaas-docs
     virtualenv -p python3 venv