Explain / warn about committeememberships in admin

Since #84 (closed) has been closed, it may be useful to add some text to the CommitteeMembership pages (perhaps above the add and edit forms) that explains how these work. In particular, the board should be instructed not to edit existing objects when chairs change, but create new objects. #84 (closed) was created because people were tempted to edit history, while they shouldn't.

Similar to #476 (closed)