OpenVR: Add note about how to enable interactions

For the interaction to work in the OpenVR examples, it is required to copy the action/manifest .json files to the working directory. This adds an according note to the description of these examples

src/Cxx/GeometricObjects/OpenVRCone.md

 ### Description
 
 This example demonstrates rendering of a Cone in OpenVR.
+
+Note that for the interactions to work in VTK >= 9.1, you will have to copy the .json manifest files from the
+subfolder Rendering/OpenVR in VTK's source to the working directory in which the tests are run!
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