netwrap: update README
netwrap: update README

Describes how to build and run.

Signed-off-by: Bogdan Pricope <7286adde866a8df2195c2226e4c247ed9ceaca28@gmail.com>
Reviewed-by: Matias Elo <62402263e8617147f0e4dd5b7f6f8ec67f2b3d2c@nokia.com>

OpenFastPath/ofp,OpenFastPath/ofp,OpenFastPath/ofp,OpenFastPath/ofp