go.mod: re-add go.mod file

Change-Id: I3aa495eccec26ec14d102d1b41901516c7238929
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/166887
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

go.mod

@@ -0,0 +1,6 @@
+module github.com/golang/protobuf/v2
+
+require (
+	github.com/golang/protobuf v1.2.1-0.20190311233832-968e039ade03
+	github.com/google/go-cmp v0.2.0
+)

go.sum

@@ -0,0 +1,4 @@
+github.com/golang/protobuf v1.2.1-0.20190311233832-968e039ade03 h1:9fMonKrK3A/zUsodyCLUQVFrEDPi/veKKdEemKA8H9k=
+github.com/golang/protobuf v1.2.1-0.20190311233832-968e039ade03/go.mod h1:6lQm79b+lXiMfvg/cZm0SGofjICqVBUtrP5yJMmIC1U=
+github.com/google/go-cmp v0.2.0 h1:+dTQ8DZQJz0Mb/HjFlkptS1FeQ4cWSnN941F8aEG4SQ=
+github.com/google/go-cmp v0.2.0/go.mod h1:oXzfMopK8JAjlY9xF4vHSVASa0yLyX7SntLO5aqRK0M=