add upgrade section on documentation

[bbmt-bbmt]

Is your feature request related to a problem? Please describe.
The actual documentation doesn't have an "how to upgrade section"
Some people may only do 'git pull' and 'git checkout version' and 'forget pip install upgrade'

Describe the solution you'd like
this command can be added to the documentation:

  $ git pull
  $ version=$(curl -s https://api.github.com/repos/pytition/pytition/releases/latest | grep "tag_name" | cut -d : -f2,3 | tr -d \" | tr -d ,)
  $ git checkout $version
  $ source pytition_venv/bin/activate
  (pytition_venv) $ pip3 install upgrade -r requirements.txt

[fallen]

Yes, good idea! Thanks :)

Also tell to:

• backup database and media first somewhere, see Write documentation #218
• re-run : collectstatic, migrate, compilemessages