Add release notes for new Sun Disk feature (#21153)

# Objective

- #20434 is really cool, and deserves a bit of a showcase.
- Fixes #21125.

## Solution

- Write release notes.
- Also add a note to the docs that this plays nicely with bloom.

---------

Co-authored-by: Emerson Coskey <emerson@coskey.dev>

Author: 2 people

crates/bevy_light/src/directional_light.rs

@@ -248,6 +248,9 @@ pub fn update_directional_light_frusta(
 /// By default, the atmosphere is rendered with [`SunDisk::EARTH`], which approximates the
 /// apparent size and brightness of the Sun as seen from Earth. You can also disable the sun
 /// disk entirely with [`SunDisk::OFF`].
+///
+/// In order to cause the sun to "glow" and light up the surrounding sky, enable bloom
+/// in your post-processing pipeline by adding a `Bloom` component to your camera.
 #[derive(Component, Clone)]
 #[require(DirectionalLight)]
 pub struct SunDisk {

release-content/release-notes/sun_disk.md

@@ -0,0 +1,22 @@
+---
+title: Procedural Sun Disk
+authors: ["@defuz"]
+pull_requests: [20434]
+---
+
+TODO: grab images from 20434 PR description.
+
+Any good [procedural atmosphere] deserves a procedural sun to light it.
+To enable this, add the [`SunDisk`] component to your [`DirectionalLight`] entity.
+The sun will move with your light, playing nicely with any positioning or movement logic you've implemented.
+
+You can set both the `angular_size` and `intensity` of the sun disk, changing the size and brightness of the sun.
+We've included a convenient `SunDisk::EARTH` constant, to spare you the tricky experimental trigonometry.
+
+If you've ever stared directly at the sun in real life (not recommended), you'll also be familiar with a spreading glow
+that bleeds out into the nearby sky.
+This is an effect known as "bloom", which is enabled by adding the [`Bloom`] component to your camera entity.
+
+[procedural atmosphere]: https://bevy.org/news/bevy-0-16/#procedural-atmospheric-scattering
+[`SunDisk`]: https://docs.rs/bevy/0.17.0-rc.1/bevy/light/struct.SunDisk.html
+[`Bloom`]: https://docs.rs/bevy/0.17.0-rc.1/bevy/post_process/bloom/struct.Bloom.html