Add note about JSON/JSONB serialization changes

The initial commit (efaa6e4) that changed this
behavior was intended to be a minor change, but ended up becoming a
large-ish breaking change within Active Record.

This is because instead of only JSON encoding `Hash`es or `Array`s in `#serialize`,
we now encode all values passed in. This is an issue if you're passing in a `String`,
that has already been transformed from a `Hash` to a `String`, since your data
is now being double encoded.

Unfortunately, the change was included in one of the v5.0.0 beta
releases, and it is too late to revert without huge ripple effects.
Thus, all we can do is update the documentation (via this commit), and
add some test coverage (coming soon in a PR) for the new behavior.

Please note that in the documentation I talk about deserialization, not
about serialization, where the actual change occurred. This is because
you won't notice any changes in serialized data until you try and
deserialize it. Also to make the change itself (confusing until you
read through everything multiple times) easier to understand.

Related rails#27788, rails#25594, rails#26101, rails#24234, rails#28292, rails#28285, rails#28285, and
probably others.

[ci skip]

guides/source/upgrading_ruby_on_rails.md

@@ -397,6 +397,15 @@ When using Ruby 2.4, you can preserve the timezone of the receiver when calling
 
     ActiveSupport.to_time_preserves_timezone = false
 
+### Changes with JSON/JSONB serialization
+
+In Rails 5.0, how JSON/JSONB attributes are serialized and deserialized changed. Now, if
+you set a column equal to a `String`, Active Record will no longer turn that string
+into a `Hash`, and will instead only return the string. This is not limited to code
+interacting with models, but also affects `:default` column settings in `db/schema.rb`.
+It is recommended that you do not set columns equal to a `String`, but pass a `Hash`
+instead, which will be converted to and from a JSON string automatically.
+
 Upgrading from Rails 4.1 to Rails 4.2
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