0.18: Add migration guide for automatic Aabb updates (#22429)

Adds migration guide for #18742. 

I'm not entirely sure it's necessary, but was implied by
#21837 (comment).

Author: greeble-dev

release-content/migration-guides/automatic_aabb_updates.md

@@ -0,0 +1,28 @@
+---
+title: Automatic `Aabb` updates for sprites and meshes
+pull_requests: [18742]
+---
+
+Bevy automatically creates an `Aabb` component for entities containing a mesh
+or sprite - the `Aabb` is then used for visibility and picking.
+
+In 0.17 the `Aabb` [was not updated](https://github.com/bevyengine/bevy/issues/4294)
+if the mesh or sprite was modified. This has been fixed in 0.18. If you were working around the issue by manually updating or removing the
+`Aabb`, then the workaround is no longer needed.
+
+```rust
+// 0.17: Modify the mesh, and remove the `Aabb` so that it's automatically
+// recreated from the modified mesh.
+mesh.insert_attribute(Mesh::ATTRIBUTE_POSITION, new_positions);
+entity.remove::<Aabb>();
+
+// 0.18: The `Aabb` will be automatically updated.
+mesh.insert_attribute(Mesh::ATTRIBUTE_POSITION, new_positions);
+```
+
+For users who want more control, 0.18 also adds a `NoAutoAabb` component. This
+will disable both automatic creation and automatic update of `Aabb` components.
+
+```rust
+entity.insert(NoAutoAabb);
+```