Improve first time experience with debusine workspace-inheritance --edit

When you run debusine workspace-inheritance $NEW_WORKSPACE --edit you get a text editor with this content:

chain: []

And you have no idea what you should add in that list. It would be nice if debusine could add a short comment at the top of the file saying something like this:

# The inheritance chain is a list of dictionaries with "id/scope/workspace" keys.
# Usually you specify the workspace and the scope. Scope is only required to
# refer to a workspace outside of the current scope. Ex:
# chain:
# - workspace: base
# - scope: some-other-scope
#   workspace: foobar

/cc @enrico