Update README.md to indicate we support up to protobuf 3.5.x
Update README.md to indicate we support up to protobuf 3.5.x

Fixes #380

scalapb/ScalaPB,scalapb/ScalaPB,trueaccord/ScalaPB,scalapb/ScalaPB,scalapb/ScalaPB,dotty-staging/ScalaPB