Describe how to sync a fork when contributing

[hoeghh]

To help beginners, we should add a section on how to sync a fork of this repo, so people can keep contributing.

Something like

# Add the remote, call it "upstream":

git remote add upstream https://github.com/jonasvinther/medusa

# Fetch all the branches of that remote into remote-tracking branches

git fetch upstream

# Make sure that you're on your master branch:

git checkout main

# Rewrite your main branch so that any commits of yours that
# aren't already in upstream/main are replayed on top of that
# other branch:

git rebase upstream/main