Add instructions when using systemctl services and python virtualenv

[NuberuSH]

When using python virtual environments to install the dependencies you need to modify the ExecStart value so it uses the correct set of libraries, but this is not specified in the how to start guide, even though it is recommended in previous steps to use them.

So, the tools/turing-smart-screen-python.service file should look something like this:

...


; If you want to put the program in your home directory instead, remove 'ProtectHome=read-only' line below
WorkingDirectory=/opt/turing-smart-screen-python/

ExecStart=/opt/turing-smart-screen-python/.venv/bin/python3 main.py

; Always restart the script
Restart=always

...

Or the proper route if you have it under the $HOME directory.

Maybe for some more experienced user this is simply logical but for people who never set services in a project using virtualenvs can be quite a headache.