Switching from Go Dep to Go Mod

go-dep-mod

1590905014

On switching from Go's `dep` tool to `mod`:

 - `dep ensure` turns into `go mod download`
 - `dep ensure -update` turns into `go-mod-upgrade` with the [go-mod-upgrade](https://github.com/oligot/go-mod-upgrade) tool
 - `dep check` turns into `go mod verify`
 - `Gopkg.toml` turns into `go.mod`
 - `Gopkg.lock` turns into `go.sum`

Add note about go mod
Add note about go mod
