Add meeting attendance policy

Our draft, proposed bylaws (!25 (closed)) include a clause saying that if a board member misses 25% or more of the meetings in the previous 12 months, they are liable to be dismissed from the board. This MR echoes this clause in the vastly more approachable board.md document.

Please consider this one carefully -- the policy described here has more bite than many.

The text in this MR was drafted by me (Richard) alone, but the idea comes from the Ways of Working committee.