CONTRIBUTING: Add "usually" wiggle to the meeting text

[wking]

This allows us to make occasional exceptions, e.g. around the holidays, without needing to adjust the text in all consuming projects. Instead, we can just ping the mailing list and bump the central iCalendar in runtime-spec (or wherever we end up keeping the central iCalendar).

The text is from the recently-landed opencontainers/runtime-spec#943 as part of synchronizing this repository with current downstream improvements (#4).