[Checkins] [zopefoundation/ZODB] e71bb8: Add a change note for #280 [skip ci]

[Jason Madden]

  Branch: refs/heads/changenote-280
  Home:   https://github.com/zopefoundation/ZODB
  Commit: e71bb8ce90f23a2a04f5d094288b8e1a4e8db52b
      https://github.com/zopefoundation/ZODB/commit/e71bb8ce90f23a2a04f5d094288b8e1a4e8db52b
  Author: Jason Madden <jamadden at gmail.com>
  Date:   2020-06-12 (Fri, 12 Jun 2020)

  Changed paths:
    M CHANGES.rst

  Log Message:
  -----------
  Add a change note for #280 [skip ci]

ConnectionPool and ConnectionPool.map both had docstrings and were used by third-party code. People should be warned about this potentially breaking change.