Add spec for the 'Automatically plan meeting' feature

While we're add it, here's another one! This time for `P1` as described in #2.
I've left out the `predefined number (probably 1 or 2) of weeks after a successful meeting` from the description of `P1` in #2; I think that's better placed with `P2`, i.e. when the system actually picks a date for a prospective meeting.