docs(gltf_extras): added small description of the newly added gltf_ex…

…tras (#1387)

Co-authored-by: Alice Cecile <[email protected]>

release-content/0.14/release-notes/13453_add_handling_of_all_missing_gltf_extras_scene_mesh__materi.md

@@ -1,4 +1,32 @@
-<!-- add handling of all missing gltf extras: scene, mesh & materials -->
-<!-- https://github.com/bevyengine/bevy/pull/13453 -->
 
-<!-- TODO -->
+The glTF 3D model file format allows passing additional user defined metadata in the *extras* properties, and
+in addition to the glTF extras at the primitive/node level , Bevy now has specific GltfExtras for:
+- scenes: **SceneGltfExtras** injected at the scene level if any
+- meshes: **MeshGltfExtras**, injected at the mesh level if any
+- materials: **MaterialGltfExtras**, injected at the mesh level if any: ie if a mesh has a material that has gltf extras, the component will be injected there.
+
+You can now easily query for these specific extras
+
+```rust
+fn check_for_gltf_extras(
+    gltf_extras_per_entity: Query<(
+        Entity,
+        Option<&Name>,
+        Option<&GltfSceneExtras>,
+        Option<&GltfExtras>,
+        Option<&GltfMeshExtras>,
+        Option<&GltfMaterialExtras>,
+    )>,
+) {
+    // use the extras' data 
+    for (id, name, scene_extras, extras, mesh_extras, material_extras) in
+        gltf_extras_per_entity.iter()
+    {
+
+    }
+}
+
+```
+
+This makes passing information from programs such as Blender to Bevy via gltf files more spec compliant, and more practical !
+