explain where .meta files came from

src/csharp/unitypackage/README.md

@@ -3,7 +3,11 @@
 Scripts in this directory are of no interest for end users. They are part
 of internal tooling to automate building of the gRPC C# package for Unity.
 
-- `unitypackage_skeleton` - preconfigured .meta files for the unity package
+- `unitypackage_skeleton` - preconfigured `.meta` files for the unity package
   layout. The actual assemblies and native libraries will be added into
   this hierarchy while building the package.
+  Note: The `.meta` file were created by the Unity IDE by manually adding the assemblies/native libraries
+  to a Unity project and configuring their target plaform/architecture in the UI (these setting get recorded in
+  `.meta` files). The `.meta` format is not very well documented and there seems to be no easy way to generate them
+  automatically.