Raise error when `source=` use on a child.

Closes #3292

Add import to example for filtering

`from rest_framework import filters` was missing from one of the examples

Fix typos in comments

Cached the field's root and context property

Resolved issue with rendering of nested serializers form when value is None

Update 2-requests-and-responses.md

just a small error, the trailing slash in the url should be omitted when adding a .suffix

Raise error if passed a serializer instance

Add a method to return the string value used in `choices` of a `RelatedField`

The `display_value` method returns a text type.

Returns the text representation of the instance. Subclasses can override this method to provide a different display value used for populating the `choices` property.

Fixed #3235 - ListField now returns the QueryDict value even if it's a list of only one item.

Fixed #3235 - A querydict which yields a list with only one value now correctly returns the provided data.

Update README with 3.2 release