aalto/jupyterhub-instructors/nbgrader-hints: Note about grade scaling

[rkdarst]

• Note a feature that's added to not scale grades to 100 when exporting.
• Also note that the email conversion isn't needed usually.
• Review: automerge

[Gehock]

LGTM, glad I happened to notice this PR just before I was about to start writing one with almost identical content.