add note on model/table

README.md

@@ -127,10 +127,10 @@ Book.eager_load(:contributors)
 Book.includes(:contributors)
 ```
 
-Right now, the underlying table for each unioned association must match. We'd
-like to change that in the future. Originally, `union_of` was defined to make
-migrating from a has-one relationship to a many-to-many relationship easier
-and safer, while retaining backwards compatibility.
+Right now, the underlying table and model for each unioned association must
+match. We'd like to change that in the future. Originally, `union_of` was
+defined to make migrating from a has-one relationship to a many-to-many
+relationship easier and safer, while retaining backwards compatibility.
 
 There is support for complex unions as well, e.g. a union made up of direct and
 through associations, even when those associations utilize union associations.