README: don't recommend go get

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: I94cd9c588a68d92c61d00be05740248509a2ae6e
Reviewed-on: https://go-review.googlesource.com/c/sys/+/624078
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Commit-Queue: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>