Update volunteer runbook with learnings

[nimbinatus]

Example:

         remember to add an "closing" section regarding load-out to storage and an "afterwards" section to schedule the volunteers lunch/dinner

maybe note something about proposing +1s for the vols social in the run-up to the conference, too, since we'll know what the budget situation is by 1 month out/1 week out... and

Originally posted by @moutons in #17 (comment)