[DOC release] Add note to embedded records mixin

As mentioned in #4575, the fact that embedded records rely on their own serializer can cause some confusion when using different serializers inside an application.

addon/serializers/embedded-records-mixin.js

@@ -12,7 +12,9 @@ var camelize = Ember.String.camelize;
 
   To set up embedded records, include the mixin when extending a serializer,
   then define and configure embedded (model) relationships.
-
+  
+  Note that embedded records will serialize with the serializer for their model instead of the serializer in which they are defined.
+  
   Below is an example of a per-type serializer (`post` type).
 
   ```app/serializers/post.js