add missing known issue

Author: vkovec

RELEASE_NOTES.md

@@ -12,6 +12,7 @@ KNOWN ISSUES
 
 * FbxExporter: animated skinned meshes must be in the bind pose on export (i.e. not being previewed in the Animation or Timeline windows, and the original rig's fbx must not contain animation)
 * FbxExporter: animated meshes in bone hierarchy are not supported
+* FbxExporter: for skinned meshes all bones must be descendants of the root bone
 * 3DIntegration: fbx containing rig must have file units in cm in order for animation exported from Unity to be properly applied
 * ConvertToPrefab: converting model instance that has been modified in the scene won't reexport fbx