Update USD Samples readme

UsdSamples/README.md

@@ -1,3 +1,10 @@
-These samples should be used with the Unity USD plugin. For Unity version higher than
-2019.1, they are distributed with the package itself and available for installation
-through the PackageManagerUI. 
+USD samples
+=================
+
+UsdSamples.unitypackage includes ready-to-use samples to allow anyone to easily jump in Unity USD integration.
+
+**This package is ONLY needed if you are running Unity versions prior to 2019.1.**
+
+Make sure you install the Unity USD plugin before installing the samples.
+
+For Unity 2019.1 and higher: please import the samples directly from Package Manager window.