message: Update README.md
  Add -cf option to the brief gltfpack overview.
diff: @@ -290,10 +290,11 @@ gltfpack -i scene.gltf -o scene.glb
  gltfpack substantially changes the glTF data by optimizing the meshes for vertex fetch and transform cache, quantizing the geometry to reduce the memory consumption and size, merging meshes to reduce the draw call count, quantizing and resampling animations to reduce animation size and simplify playback, and pruning the node tree by removing or collapsing redundant nodes. It will also simplify the meshes when requested to do so.
  -gltfpack can produce two types of output files:
  +gltfpack can produce three types of output files:
  - By default gltfpack outputs regular `.glb`/`.gltf` files that have been optimized for GPU consumption using various cache optimizers and quantization. These files can be loaded by standard GLTF loaders present in frameworks such as [three.js](https://threejs.org/) (r107+) and [Babylon.js](https://www.babylonjs.com/) (4.1+).
  - When using `-c` option, gltfpack outputs compressed `.glb`/`.gltf` files that use meshoptimizer codecs to reduce the download size further. Loading these files requires extending GLTF loaders with custom decompression support; `demo/GLTFLoader.js` contains a custom version of three.js loader that can be used to load them.
  +- When using `-cf` option, gltfpack outputs compressed files and an extra `.fallback.bin` file with uncompressed data. These files can be loaded by standard glTF loaders; loaders with decompression support don't need to load the fallback.
  > Note: files produced by gltfpack use `MESHOPT_quantized_geometry` and `MESHOPT_compression` pseudo-extensions; both of these have *not* been standardized yet but eventually will be. glTF validator doesn't recognize these extensions and produces a large number of validation errors because of this.
  