README: add compatibility section
README: add compatibility section

golang/protobuf,fuchsia-mirror/third_party-github.com-golang-protobuf,tamird/protobuf-2,skyportsystems/go-protobuf,golang/protobuf,tamird/protobuf-2,skyportsystems/go-protobuf,trdata/protobuf,fuchsia-mirror/third_party-github.com-golang-protobuf,trdata/protobuf