go.mod: add module file (#94)
go.mod: add module file (#94)

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.
google/go-cmp