Document weekend shift swaps

Why is this change being made?

For folks who have not yet participated in the IMOC rotation, there is confusion around being scheduled for a weekend shift when local laws may prohibit that. This is something we have worked through with existing incident managers, but is not documented.

This MR is just to document what we have already been doing, but leaves space for us to iterate in the future by perhaps considering a separate scheduling layer that excludes weekend shifts depending on country or some other method.

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