all: add a go.mod file

This change adds a go.mod file to this repo, following the
requirements stated in bcmills's comment here:
https://golang.org/issue/28136#issuecomment-462971974. It's
important to note that we will not be
adding versions to the repo for now.

There is no go.sum file because this repo has no dependencies.

The change was generated by running "go mod init" and "go mod tidy" using Go
1.11.5 in the top-level directory of the repository.

Updates golang/go#28136

Change-Id: Ie24d8bb73104e8299d3d22cf6c72a295036804f7
Reviewed-on: https://go-review.googlesource.com/c/review/+/162916
Run-TryBot: Michael Matloob <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>

go.mod

@@ -0,0 +1 @@
+module golang.org/x/review