Add mesh tag feature to release notes. (#2081)

Author: tychedelia

release-content/0.16/release-notes/17648_mesh_tags.md

@@ -0,0 +1,32 @@
+Bevy has powerful support for [automatic instancing] for any entities that share the same mesh and material. However, sometimes it can still be useful to reference data that is not the same across all instances of a material. Previously, this required either writing significant amount of custom rendering code or giving up the performance benefits of automatic instancing by creating more materials.
+
+The new `MeshTag` component allows adding a custom `u32` tag to mesh-material entities that can be referenced in the vertex shader for a material. In combination with storage textures or the [`ShaderStorageBuffer` asset] added in Bevy 0.15, this provides a flexible new mechanism to access external data on a per-instance basis or otherwise tag your mesh instances.
+
+Spawn a mesh tag with a mesh-material entity: 
+```rust
+commands.spawn((
+    // Clone a mesh and material handle to enable automatic instancing 
+    Mesh3d(mesh_handle.clone()),
+    MeshMaterial3d(material_handle.clone()),
+    // The mesh tag can be any `u32` that is meaningful to your application, like
+    // a particular variant of an enum or an index into some external data 
+    MeshTag(1234), 
+));
+```
+
+Lookup the tag in a vertex shader:
+```wgsl
+#import bevy_pbr::mesh_functions
+
+@vertex
+fn vertex(vertex: Vertex) -> VertexOutput {
+    // Lookup the tag for the given mesh
+    let tag = mesh_functions::get_tag(vertex.instance_index);
+
+    // Index into a storage buffer, read a storage texture texel, etc...
+}
+
+```
+
+[automatic instancing]: https://bevyengine.org/examples/shaders/automatic-instancing/
+[`ShaderStorageBuffer` asset]: https://bevyengine.org/news/bevy-0-15/#shader-storage-buffer-asset

release-content/0.16/release-notes/_release-notes.toml

@@ -148,6 +148,13 @@ contributors = ["@atlv24"]
 prs = [17765, 16947, 16941]
 file_name = "virtual_geometry_improvements.md"
 
+[[release_notes]]
+title = "Mesh tags"
+authors = ["@tychedelia"]
+contributors = []
+prs = [17648]
+file_name = "17648_mesh_tags.md"
+
 # UI
 
 [[release_notes]]