These days people will just import the packages and the go tool will
do the right thing. We don't need to explain it.

Add a pointer to the git repo, though.

For golang/go#62645

Change-Id: Ib6a6fb8989df8071b20d50de446d05b270c1f1ae
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/624195
Commit-Queue: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cody Oss <codyoss@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

Change-Id: I9fd025393acc12da59fdac1c416563324cd26af1
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/639695
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>