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:
46492949b8

guides/source/upgrading_ruby_on_rails.md

@@ -883,6 +883,18 @@ this gem such as `whitelist_attributes` or `mass_assignment_sanitizer` options.
 
 * To re-enable the old finders, you can use the [activerecord-deprecated_finders gem](https://github.com/rails/activerecord-deprecated_finders).
 
+* Rails 4.0 has changed to default join table for `has_and_belongs_to_many` relations to strip the common prefix off the second table name. Any existing `has_and_belongs_to_many` relationship between models with a common prefix must be specified with the `join_table` option. For example:
+
+```ruby
+CatalogCategory < ActiveRecord::Base
+  has_and_belongs_to_many :catalog_products, join_table: 'catalog_categories_catalog_products'
+end
+
+CatalogProduct < ActiveRecord::Base
+  has_and_belongs_to_many :catalog_categories, join_table: 'catalog_categories_catalog_products'
+end
+```
+
 ### Active Resource
 
 Rails 4.0 extracted Active Resource to its own gem. If you still need the feature you can add the [Active Resource gem](https://github.com/rails/activeresource) in your Gemfile.