Update what's new

dist/preview release/what's new.md

@@ -99,6 +99,7 @@
 - Added debug logging and performance counters ([bghgary](http://www.github.com/bghgary))
 - Added support for EXT_lights_imageBased ([bghgary](http://www.github.com/bghgary))
 - Added support for MSFT_audio_emitter ([najadojo](http://www.github.com/najadojo))
+- Added support for custom loader extensions ([bghgary](http://www.github.com/bghgary))
 
 ### Viewer