Improve handling of events starting in the past

[pdinklag]

Currently, events are supposed to start in the future, so there is at least one update where an event is already configured, but has not yet started. Only then will initial rankings be computed properly, because if the event has already started, there is no telling with what scores players started into the event.

Right now, the behaviour is that if an event starts in the past, the initial score is assumed to be zero, which does not really represent the reality well. Instead, if an event starts in the past, the following should be done:

• Use the current scores as the initial scores.
• Emit a warning, noting that the event scores may be off since scores achieved the last update and the event start could not be considered.

[pdinklag]

I've decided this is more of a documentation issue rather than anything else. Events should be setup so that at least an update occurs before they begin, and the readme should state so.

[pdinklag]

Documented in v3.0.0.