go.mod: add module file (#699)

Go 1.11 is now released with preliminary support for Go modules.
If a repository is checked outside of the GOPATH, then the Go toolchain will
use the module support by default. However, it requires a go.mod file so that
the toolchain can derive the import paths of all packages living under the
tree containing the go.mod file.

go.mod

@@ -0,0 +1,7 @@
+module github.com/golang/protobuf
+
+require (
+	golang.org/x/net v0.0.0-20180906233101-161cd47e91fd // indirect
+	golang.org/x/sync v0.0.0-20180314180146-1d60e4601c6f
+	google.golang.org/genproto v0.0.0-20180831171423-11092d34479b
+)

go.sum

@@ -0,0 +1,6 @@
+golang.org/x/net v0.0.0-20180906233101-161cd47e91fd h1:nTDtHvHSdCn1m6ITfMRqtOd/9+7a3s8RBNOZ3eYZzJA=
+golang.org/x/net v0.0.0-20180906233101-161cd47e91fd/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
+golang.org/x/sync v0.0.0-20180314180146-1d60e4601c6f h1:wMNYb4v58l5UBM7MYRLPG6ZhfOqbKu7X5eyFl8ZhKvA=
+golang.org/x/sync v0.0.0-20180314180146-1d60e4601c6f/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+google.golang.org/genproto v0.0.0-20180831171423-11092d34479b h1:lohp5blsw53GBXtLyLNaTXPXS9pJ1tiTw61ZHUoE9Qw=
+google.golang.org/genproto v0.0.0-20180831171423-11092d34479b/go.mod h1:JiN7NxoALGmiZfu7CAH4rXhgtRTLTxftemlI0sWmxmc=