add upgrade note for breaking habtm change in 3.2 -> 4.0
add upgrade note for breaking habtm change in 3.2 -> 4.0

The has_and_belongs_to_many default join_table behavior changed
between Rails 3.2 and Rails 4.0 and should be noted in the
appropriate section of the upgrade guide.

commit:
https://github.com/rails/rails/commit/46492949b8c09f99db78b9f7a02d039e7bc6a702

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