update known issues

Author: davidkline-ms

Documentation/ReleaseNotes.md

@@ -28,6 +28,10 @@ There are no breaking changes since version 2.5.0.
 
 ## Known issues
 
+### Some Mixed Reality Toolkit Standard Shader features require the Foundation package
+
+When imported via the Unity Package Manager, the MRTK Standard Shader utilities scripts (ex: HoverLight.cs) are not co-located with the shader in the Standard Assets package. To access this functionality, applications will require the Foundation package to be imported.
+
 ### CameraCache may create a new camera on shutdown
 
 In some situations (e.g. when using the LeapMotion provider in the Unity Editor), it is possible for the CameraCache to re-create the MainCamera on shutdown. Please see [this issue](https://github.com/microsoft/MixedRealityToolkit-Unity/issues/8459) for more information.