Merge pull request #692 from lexaknyazev/texture-origin

Add clarification on texture origin.

specification/2.0/README.md

@@ -960,6 +960,7 @@ Images referred to by textures are stored in the `images` dictionary property of
     }
 },
 ```
+> **Implementation Note**: With WebGL API, the first pixel transferred from the `TexImageSource` (i.e., HTML Image object) to the WebGL implementation corresponds to the upper left corner of the source. Non-WebGL runtimes may need to flip Y axis to achieve correct texture rendering.
 
 #### Samplers