_examples: submodule, adding an example with submodule update

the goal of this pull request is to add an example of how to make
the equivalent of `git submodule update --remote` with `go-git`

_examples/submodule/main.go

+package main
+
+import (
+	"os"
+
+	"github.com/go-git/go-git/v5"
+	. "github.com/go-git/go-git/v5/_examples"
+)
+
+// Basic example of how to clone a repository including a submodule and
+// updating submodule ref
+func main() {
+	CheckArgs("<url>", "<directory>", "<submodule>")
+	url := os.Args[1]
+	directory := os.Args[2]
+	submodule := os.Args[3]
+
+	// Clone the given repository to the given directory
+	Info("git clone %s %s --recursive", url, directory)
+
+	r, err := git.PlainClone(directory, false, &git.CloneOptions{
+		URL:               url,
+		RecurseSubmodules: git.DefaultSubmoduleRecursionDepth,
+	})
+
+	CheckIfError(err)
+
+	w, err := r.Worktree()
+	if err != nil {
+		CheckIfError(err)
+	}
+
+	sub, err := w.Submodule(submodule)
+	if err != nil {
+		CheckIfError(err)
+	}
+
+	sr, err := sub.Repository()
+	if err != nil {
+		CheckIfError(err)
+	}
+
+	sw, err := sr.Worktree()
+	if err != nil {
+		CheckIfError(err)
+	}
+
+	Info("git submodule update --remote")
+	err = sw.Pull(&git.PullOptions{
+		RemoteName: "origin",
+	})
+	if err != nil {
+		CheckIfError(err)
+	}
+}