groot: document interop model with ROOT/C++
groot: document interop model with ROOT/C++

Fixes go-hep/hep#947.

Signed-off-by: Sebastien Binet <be0c0d1298e63814a95fcb7523d1ca1345b4ca1f@cern.ch>

go-hep/hep,go-hep/hep