Clean up grpc spec for the merged project (#160)
Clean up grpc spec for the merged project (#160)

* initial move of a file

* clean up grpc doc

* note about existing integrations

* move back so it's easier to compare

* Update gRPC.md

open-telemetry/opentelemetry-java,open-telemetry/opentelemetry-java,open-telemetry/opentelemetry-java