_examples: Add call to config example

Signed-off-by: Paulo Gomes <[e-mail]>

_examples/README.md

···
       a repository using a GitHub personal access token.
     - [ssh private key](clone/auth/ssh/private_key/main.go) - Cloning a repository using a ssh private key.
     - [ssh agent](clone/auth/ssh/ssh_agent/main.go) - Cloning a repository using ssh-agent.
+- [config](config/main.go) - Explains how to work with config files. 
 - [commit](commit/main.go) - Commit changes to the current branch to an existent repository.
 - [push](push/main.go) - Push repository to default remote (origin).
 - [pull](pull/main.go) - Pull changes from a remote repository.

_examples/common_test.go

···
 	"checkout":                   {defaultURL, tempFolder(), "35e85108805c84807bc66a02d91535e1e24b38b9"},
 	"checkout-branch":            {defaultURL, tempFolder(), "branch"},
 	"clone":                      {defaultURL, tempFolder()},
+	"config":                     {},
 	"commit":                     {cloneRepository(defaultURL, tempFolder())},
 	"context":                    {defaultURL, tempFolder()},
 	"custom_http":                {defaultURL},

_examples/config/main.go

···
 package main
 
 import (
+	"os"
+
 	"github.com/go-git/go-git/v5"
 	. "github.com/go-git/go-git/v5/_examples"
 
···
 // - Set basic local config params
 
 func main() {
+	tmp, err := os.MkdirTemp("", "go-git-example")
+	CheckIfError(err)
+	defer os.RemoveAll(tmp)
+
 	Info("git init")
-	r, err := git.PlainInit(".", false)
+	r, err := git.PlainInit(tmp, false)
 	CheckIfError(err)
 
 	// Load the configuration