Update README.md

Clarifications for glTF Embedded, for more details see KhronosGroup/glTF#1915

README.md

@@ -36,7 +36,7 @@ A summary of the model license is shown in each display, but see the `README.md`
 Sample glTF 2.0 models are provided in one or more of the following forms of glTF:
 
 * glTF (`.gltf`) with separate resources: `.bin` (geometry, animation, skins) and `.jpg` or `.png` image files.  The supporting files are easily examined when separated like this, but must be kept together with the parent glTF file for the model to work.
-* glTF (`.gltf`) with embedded resources (as Data URIs).  This form tends to be larger than the others, but Data URIs do have their uses.
+* glTF Embedded (`.gltf`) with embedded resources as Data URIs.  This form is intended for very small binary payloads, similar to the use of small data URIs in HTML/CSS.  This form should be avoided except as needed for a specific use case.
 * Binary glTF (`.glb`) using the [binary container format](https://github.com/KhronosGroup/glTF/blob/master/specification/2.0/README.md#glb-file-format-specification).  These are easily shared due to the bundling of all the textures and mesh data into a single file.