Add KHR_materials_emissive_strength to readme

Author: UX3D-kanzler

README.md

@@ -36,6 +36,7 @@ Features
 - [x] [KHR_draco_mesh_compression](https://github.com/KhronosGroup/glTF/tree/master/extensions/2.0/Khronos/KHR_draco_mesh_compression)
 - [x] [KHR_lights_punctual](https://github.com/KhronosGroup/glTF/tree/master/extensions/2.0/Khronos/KHR_lights_punctual)
 - [x] [KHR_materials_clearcoat](https://github.com/KhronosGroup/glTF/tree/master/extensions/2.0/Khronos/KHR_materials_clearcoat)
+- [x] [KHR_materials_emissive_strength](https://github.com/KhronosGroup/glTF/tree/KHR_materials_emissive_strength/extensions/2.0/Khronos/KHR_materials_emissive_strength)
 - [x] [KHR_materials_pbrSpecularGlossiness](https://github.com/KhronosGroup/glTF/tree/master/extensions/2.0/Khronos/KHR_materials_pbrSpecularGlossiness)
 - [x] [KHR_materials_ior](https://github.com/KhronosGroup/glTF/tree/master/extensions/2.0/Khronos/KHR_materials_ior)
 - [x] [KHR_materials_sheen](https://github.com/KhronosGroup/glTF/tree/master/extensions/2.0/Khronos/KHR_materials_sheen)