[Checkins] [zopefoundation/Zope] cb3a3a: Add a bit of migration documentation.

[Michael Howitz]

  Branch: refs/heads/support-python3.9
  Home:   https://github.com/zopefoundation/Zope
  Commit: cb3a3a27b7c9c3bcd4800f8001de73de8dabe5eb
      https://github.com/zopefoundation/Zope/commit/cb3a3a27b7c9c3bcd4800f8001de73de8dabe5eb
  Author: Michael Howitz <mh at gocept.com>
  Date:   2020-10-07 (Wed, 07 Oct 2020)

  Changed paths:
    M docs/migrations/index.rst
    A docs/migrations/zope5.rst

  Log Message:
  -----------
  Add a bit of migration documentation.