Update release notes

docs/releases/3.0.md

@@ -56,6 +56,42 @@ class LandingPage(Page):
 
 Trying to upload an image that's a duplicate of one already in the image library will now lead to a confirmation step. This feature was developed by Tidiane Dia and sponsored by The Motley Fool.
 
+### Image renditions can now be prefetched
+
+When using a queryset to render a list of items with images, you can now make use of Django's built-in ``prefetch_related()`` queryset method to prefetch the renditions needed for rendering with a single extra query. For long lists of items, or where multiple renditions are used for each item, this can provide a significant boost to performance.
+
+For example, say you were rendering a list of events (with thumbnail images for each). The queryset for your source data might look something like this:
+
+```python
+EventPage.objects.live().select_related("listing_image")
+```
+
+Now that renditions are prefetchable, you can reduce the number of additional database queries needed for rendering by updating the original queryset like so:
+
+```python
+EventPage.objects.live().select_related("listing_image").prefetch_related("listing_image__renditions")
+```
+
+If each image in your project has large numbers of renditions, you might want to consider using a ``Prefetch`` object to select only the renditions you need. For example:
+
+```python
+from django.db.models import Prefetch
+from wagtail.images import get_image_model
+
+
+# These are the renditions required for rendering
+renditions = get_image_model().get_rendition_model().objects.filter(
+    filter_spec__in=["fill-300x186", "fill-600x400", "fill-940x680"]
+)
+
+# `Prefetch` is used to only fetch the required renditions
+events = EventPage.objects.live().select_related("listing_image").prefetch_related(
+    Prefetch("listing_image__renditions", queryset=renditions)
+)
+```
+
+This feature was developed by Andy Babic.
+
 ### Other features
 
  * Upgrade ESLint and Stylelint configurations to latest shared Wagtail configs (Thibaud Colas, Paarth Agarwal)