Initial link from Serializers to Validators

docs/api-guide/serializers.md

@@ -505,6 +505,19 @@ This option should be a list or tuple of field names, and is declared as follows
 
 Model fields which have `editable=False` set, and `AutoField` fields will be set to read-only by default, and do not need to be added to the `read_only_fields` option.
 
+---
+
+**Note**: There is a special-case where a read-only field is part of a `unique_together` constraint at the model level. Here you **must** specify the field explicitly and provide a valid default value.
+
+A common example of this is a read-only relation to currently authenticated `User` which is `unique_together` with another identifier. In this case you would declare the user field like so:
+
+    user = serializers.PrimaryKeyRelatedField(read_only=True, default=serializers.CurrentUserDefault())
+
+Please review the [Validators Documentation](/api-guide/validators/) for details on the [UniqueTogetherValidator](/api-guide/validators/#uniquetogethervalidator) and [CurrentUserDefault](/api-guide/validators/#currentuserdefault) classes.
+
+---
+
+
 ## Specifying additional keyword arguments for fields.
 
 There is also a shortcut allowing you to specify arbitrary additional keyword arguments on fields, using the `extra_kwargs` option. Similarly to `read_only_fields` this means you do not need to explicitly declare the field on the serializer.