Add new language defining mappings usage within viewer applications

Author: jercytryn

extensions/2.0/Khronos/KHR_materials_variants/README.md

@@ -108,7 +108,9 @@ In the second case, the shoe would have a white body and sole but red laces. In
 ]
 ```
 
-The variants property within each mapping refers to the index of a material variant located at the glTF root node:
+Viewer-specific logic defines, for a given instance of a glTF asset, up to one single active variant. Each `mappings` item gives the material to a compliant viewer should apply for that primitive when any of its `variants` indices is the active one.  If there are no matching variants found within `mappings`, or there is no active variant, fall back on vanilla glTF behaviour. Across the entire `mappings` array, each variant index and each material must be used no more than one time.
+
+For each mapping, each index of the `variants` property refers to the index of a material variant located at the glTF root node:
 
 ```javascript
 {