Document the need for graphql_fields on custom rendition models

[zerolab]

When using a custom image model, with a custom rendition model one needs to add

 graphql_fields = [
        GraphQLString("id"),
        GraphQLString("url"),
        GraphQLString("width"),
        GraphQLString("height"),
        GraphQLImage("image"),
        GraphQLString("file"),
    ]

to the custom rendition model. (Alternatively

wagtail-grapple/example/images/models.py

Lines 16 to 35 in 5669676

	class CustomImageRendition(AbstractRendition):
	image = models.ForeignKey(
	"CustomImage", related_name="renditions", on_delete=models.CASCADE
	)
	class Meta:
	unique_together = (("image", "filter_spec", "focal_point_key"),)
	def custom_rendition_property(self, info, **kwargs):
	return "Rendition Model!"
	graphql_fields = (
	GraphQLString("custom_rendition_property", required=True),
	GraphQLInt("id", required=True),
	GraphQLString("url", required=True),
	GraphQLString("width", required=True),
	GraphQLString("height", required=True),
	GraphQLImage("image", required=True),
	GraphQLString("file", required=True),
	)

)

Document, or fix so it is not needed