Change Readme to use Venv

[athyfr]

Popularly used Debian systems do not support installing programs using pip without a container.

I think it would be a good idea to update the README to reflect the common practice of using containers (using venv)

So this:

pip install -r requirements.txt
python3 trollstore.py

might be replaced by:

python3 -m venv trollstore-venv
source trollstore-venv/bin/activate
pip install -r requirements.txt
python3 trollstore.py