Add rule about top 15 co-op runs to F.3 video rules

[audiosalix]

speedrun.com category rules includes the rule "For competitive (top fifteen) times, videos of all perspectives are required for 2-4 player runs, four perspectives for 5-9 player runs, and five for 10+." which is absent from the official ruleset

[audiosalix]

maybe lower threshold than top fifteen, that's a pretty high % of the runs for less contested categories