doc improvement

[sloriot]

• $HOME/.config/CGAL/test_cgal_rc is not documented
• Do we still need --user and --passwd now that internal release are publicly accessible
  • If we don't then we can publish the complete config and cron for our test plateforms
• Do we need to do docker pull before being able to use test_cgal.py?
• Mentioning https://docs.docker.com/engine/install/linux-postinstall/#manage-docker-as-a-non-root-user might be useful
• about xdg, we needed to do pip2 install pyxdg and not xdg that was generating an error at runtime
• about docker, latest version does not seem to work pip2 install docker==3.5.0
• is the script compatible with python3?

[SaillantNicolas]

• to use pip2 instend of standard pip python2 -m ensurepip