Add example to README.md

Author: sarkahn

examples/README.md

@@ -107,6 +107,7 @@ Example | File | Description
 `update_gltf_scene` | [`3d/update_gltf_scene.rs`](./3d/update_gltf_scene.rs) | Update a scene from a gltf file, either by spawning the scene as a child of another entity, or by accessing the entities of the scene
 `wireframe` | [`3d/wireframe.rs`](./3d/wireframe.rs) | Showcases wireframe rendering
 `z_sort_debug` | [`3d/z_sort_debug.rs`](./3d/z_sort_debug.rs) | Visualizes camera Z-ordering
+`custom_mesh` | [`3d/custom_mesh.rs`](./3d/custom_mesh.rs) | Simple example showing how to manually create a custom mesh.
 
 ## Application