Add example of pure HS3, and ROOT

[mmikhasenko]

It would be great to advirtise HS3 and ROOT implementation.
Even tho, no amplitude models can be read, a simple example of exponent + gauss would do the job.

Once python bindings work for RooJSONFactoryWSTool, it should be easy with pyroot. See hep-statistics-serialization-standard/hep-statistics-serialization-standard#42