Suggestion: Add venv recommendation to installation instructions

[JulioLopio]

Hello,
first of all, thank you for your great work on SpiderFoot — it's a fantastic OSINT tool!

I noticed that when installing SpiderFoot on current Ubuntu 24.04 LTS (with Python 3.12), trying to run:

pip install -r requirements.txt

outside of a virtual environment results in this error:

error: externally-managed-environment

This is due to the new "externally-managed" policy in Python 3.12, which is now default in many distributions.

I would like to suggest adding a short recommendation to the installation instructions (README), e.g.:

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