Examples: Checkout Commit

[marwan-at-work]

I think it would be great to add as many examples to the repo as possible, given that workflows are different. i.e.:

git cli: git checkout ae200dcc6592716c757372cf01825f21a6b6xxxx
go-git, sudo, : repo.Worktree().Checkout(plumbing.NewHash("ae200dcc6592716c757372cf01825f21a6b6xxxx")

Although this might be trivial to git-pros, I think it's really good to include common scenarios like this one. I'd love to submit a quick pr for it. Here's a simple example:

repo, err := git.PlainClone("/tmp/foo", false, &git.CloneOptions{
  URL: "https://github.com/src-d/go-git.git",
  Progress: os.Stdout,
})
CheckIfError(err)

wt, err := repo.Worktree()
CheckIfError(err)

commitRef := plumbing.NewHash("ae200dcc6592716c757372cf01825f21a6b6xxxx")
err = wt.Checkout(commitRef)
CheckIfError(err)

[smola]

@marwan-at-work We'll be glad to accept a PR adding a checkout example.

[marwan-at-work]

#331
cc: @smola