Add release note for atmosphere env map (bevyengine#20627)

# Objective

- Follow up on the PR to add the release note
- keep a friendly tone with no technical details (they are in the PR)
- focus on the use cases rather than implementation details

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>

Author: mate-h

release-content/release-notes/atmosphere_environment_map.md

@@ -0,0 +1,25 @@
+---
+title: Generated environment map for procedural atmosphere
+authors: ["@mate-h"]
+pull_requests: [20529]
+---
+
+(TODO: Embed screenshot of atmosphere-generated reflections)
+
+You can now have dynamic reflections and ambient light in your scene that match the procedural sky.
+
+As the sky changes, reflections on shiny and rough materials update automatically to stay consistent — no pre-baked environment maps needed.
+
+To enable this for a camera, add the new component `AtmosphereEnvironmentMapLight` to the camera entity:
+
+```rust
+commands.spawn((
+    Camera3d::default(),
+    // Generates an environment cubemap from the atmosphere for this view
+    AtmosphereEnvironmentMapLight::default(),
+));
+```
+
+Note that this is a per-view effect (per camera). Light probes are not yet supported.
+
+Special thanks to @atlv24, @JMS55 and @ecoskey for reviews and feedback.