Add comments to python venv instructions

Signed-off-by: Jeff Goeders <[email protected]>

Author: jgoeders

BUILDING.md

@@ -18,7 +18,7 @@ Then to install the Python packages:
 
     pip install -r requirements.txt
 
-**Note:** If you chose to install the Python virtual environment, you will need to remember to activate it on any new terminal window you use, before you can run the VTR flow or regressions tests.
+**Note:** If you chose to install the Python virtual environment, you will need to remember to activate it on any new terminal window you use, before you can run the VTR flow or regressions tests (`source .venv/bin/activate`).
 
 ## Building
 

doc/src/quickstart/index.rst

@@ -23,13 +23,13 @@ VTR requires several system packages and Python packages to build and run the fl
 
     > ./install_apt_packages.sh
 
-Then, to install the required Python packages within a new virtual environment:
+Then, to install the required Python packages (optionally within a new Python virtual environment):
 
 .. code-block:: bash
 
-    > make env
-    > source .venv/bin/activate
-    > pip install -r requirements.txt
+    > make env                          # optional: install python virtual environment
+    > source .venv/bin/activate         # optional: activate python virtual environment
+    > pip install -r requirements.txt   # install python packages (in virtual environment if prior commands run, system wide otherwise)
 
 
 Build VTR